Relax text validation to support empty strings in multimodal scenarios

[RuiZhang-cn]

code:

org.springframework.ai.chat.client.DefaultChatClient.DefaultPromptUserSpec
		@Override
		public PromptUserSpec text(String text) {
			Assert.hasText(text, "text cannot be null or empty");
			this.text = text;
			return this;
		}

In DefaultChatClient.DefaultPromptUserSpec.text(), the current Assert.hasText(text) validation rejects empty strings. However, when calling the vision model, it is possible to pass an empty string to indicate that the user has only provided an image. For example, the following curl request works correctly:

curl * \
  -H "Content-Type: application/json" \
  -H "Authorization: Bearer *" \
  -d '{
    "model": "gpt-4o",
    "messages": [
      {
        "role": "user",
        "content": [
          {
            "type": "text",
            "text": ""
          },
          {
            "type": "image_url",
            "image_url": {
              "url": "https://upload.wikimedia.org/wikipedia/commons/thumb/d/dd/Gfp-wisconsin-madison-the-nature-boardwalk.jpg/2560px-Gfp-wisconsin-madison-the-nature-boardwalk.jpg"
            }
          }
        ]
      }
    ]
  }'

Proposed Fix:
Modify the validation to allow empty strings while still rejecting null values:

Assert.notNull(text, "text cannot be null"); // Keep null check
this.text = text; // Allow empty string