[cli_config] Fix optionalString validValues (#69)

Author: dcharkes

pkgs/cli_config/lib/src/config.dart

@@ -225,7 +225,7 @@ class Config {
     for (final source in _sources) {
       value ??= source.optionalString(key);
     }
-    if (validValues != null) {
+    if (value != null && validValues != null) {
       Source.throwIfUnexpectedValue(key, value, validValues);
     }
     return value;

pkgs/cli_config/test/cli_config_test.dart

@@ -309,6 +309,11 @@ void main() {
     );
   });
 
+  test('optionalString validValues', () {
+    final config = Config();
+    expect(config.optionalString('foo', validValues: ['bar']), isNull);
+  });
+
   test('valueOf file source', () {
     final config = Config(fileParsed: {
       'key': {'some': 'map'}