Python fix native function default parameter (#2474)

### Motivation and Context

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solve #2473  and add unit test to prevent this from happening again. 

### Description

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### Contribution Checklist

<!-- Before submitting this PR, please make sure: -->

- [ ] The code builds clean without any errors or warnings
- [ ] The PR follows the [SK Contribution
Guidelines](https://github.com/microsoft/semantic-kernel/blob/main/CONTRIBUTING.md)
and the [pre-submission formatting
script](https://github.com/microsoft/semantic-kernel/blob/main/CONTRIBUTING.md#development-scripts)
raises no violations
- [ ] All unit tests pass, and I have added new tests where possible
- [ ] I didn't break anyone 😄

---------

Co-authored-by: Abby Harrison <[email protected]>

Author: joowon-dm-snu

python/semantic_kernel/orchestration/sk_function.py

@@ -82,6 +82,7 @@ def from_native_method(method, skill_name="", log=None) -> "SKFunction":
         if (
             hasattr(method, "__sk_function_input_description__")
             and method.__sk_function_input_description__ is not None
+            and method.__sk_function_input_description__ != ""
         ):
             input_param = ParameterView(
                 "input",

python/tests/unit/orchestration/test_native_function.py

@@ -0,0 +1,100 @@
+# Copyright (c) Microsoft. All rights reserved.
+
+from semantic_kernel.orchestration.sk_function import SKFunction
+from semantic_kernel.skill_definition.sk_function_decorator import sk_function
+
+
+def test_init_native_function_with_input_description():
+    class MockMethodWithInputDescription:
+        __sk_function__ = True
+        __sk_function_name__ = "mock_function"
+        __sk_function_description__ = "Mock description"
+        __sk_function_input_description__ = "Mock input description"
+        __sk_function_input_default_value__ = "default_input_value"
+        __sk_function_context_parameters__ = [
+            {
+                "name": "param1",
+                "description": "Param 1 description",
+                "default_value": "default_param1_value",
+            }
+        ]
+
+    mock_method = MockMethodWithInputDescription
+
+    native_function = SKFunction.from_native_method(mock_method, "MockSkill")
+
+    assert native_function._function == mock_method
+    assert native_function._parameters[0].name == "input"
+    assert native_function._parameters[0].description == "Mock input description"
+    assert native_function._parameters[0].default_value == "default_input_value"
+    assert native_function._parameters[1].name == "param1"
+    assert native_function._parameters[1].description == "Param 1 description"
+    assert native_function._parameters[1].default_value == "default_param1_value"
+
+
+def test_init_native_function_without_input_description():
+    class MockMethodWithoutInputDescription:
+        __sk_function__ = True
+        __sk_function_name__ = "mock_function_no_input_desc"
+        __sk_function_description__ = "Mock description no input desc"
+        __sk_function_context_parameters__ = [
+            {
+                "name": "param1",
+                "description": "Param 1 description",
+                "default_value": "default_param1_value",
+            }
+        ]
+
+    mock_method = MockMethodWithoutInputDescription
+
+    native_function = SKFunction.from_native_method(mock_method, "MockSkill")
+
+    assert native_function._function == mock_method
+    assert native_function._parameters[0].name == "param1"
+    assert native_function._parameters[0].description == "Param 1 description"
+    assert native_function._parameters[0].default_value == "default_param1_value"
+
+
+def test_init_native_function_from_sk_function_decorator():
+    @sk_function(
+        description="Test description",
+        name="test_function",
+        input_description="Test input description",
+        input_default_value="test_default_value",
+    )
+    def decorated_function():
+        pass
+
+    assert decorated_function.__sk_function__ is True
+    assert decorated_function.__sk_function_description__ == "Test description"
+    assert decorated_function.__sk_function_name__ == "test_function"
+    assert (
+        decorated_function.__sk_function_input_description__ == "Test input description"
+    )
+    assert (
+        decorated_function.__sk_function_input_default_value__ == "test_default_value"
+    )
+
+    native_function = SKFunction.from_native_method(decorated_function, "MockSkill")
+
+    assert native_function._function == decorated_function
+    assert native_function._parameters[0].name == "input"
+    assert native_function._parameters[0].description == "Test input description"
+    assert native_function._parameters[0].default_value == "test_default_value"
+
+
+def test_init_native_function_from_sk_function_decorator_defaults():
+    @sk_function()
+    def decorated_function():
+        pass
+
+    assert decorated_function.__sk_function__ is True
+    assert decorated_function.__sk_function_description__ == ""
+    assert decorated_function.__sk_function_name__ == "decorated_function"
+    assert decorated_function.__sk_function_input_description__ == ""
+    assert decorated_function.__sk_function_input_default_value__ == ""
+
+    native_function = SKFunction.from_native_method(decorated_function, "MockSkill")
+
+    assert native_function._function == decorated_function
+    assert len(native_function._parameters) == 0