Skip empty text content blocks for Bedrock user messages

[chabinhwang]

Summary

BedrockProxyChatModel always added a text ContentBlock for a UserMessage, even when the message had no text.
The Converse API rejects an empty text content block, so a user message that carries only media failed with an HTTP 400.

The assistant message branch of the same method already guards with StringUtils.hasText(...); this applies the same guard to the user message branch.

Fixes #6695

Reproduction

From the issue: a prompt whose user message has text("") and one image, sent to a vision-capable model through Bedrock, is rejected by the API. The same request without the empty text block succeeds.

UserMessage.builder()
    .text("")
    .media(Media.builder().mimeType(MediaType.IMAGE_PNG).data(pngBytes).build())
    .build()

Testing

Three tests added to BedrockProxyChatModelTest, all going through createRequest, so they assert on the actual ConverseRequest payload:

• mediaOnlyUserMessageOmitsEmptyTextContentBlock — empty text plus image yields a single image block and no text block
• blankUserMessageTextOmitsEmptyTextContentBlock — whitespace-only text is treated the same way
• userMessageWithTextAndMediaKeepsBothContentBlocks — regression guard: text plus image still yields both blocks in order

Verified locally:

• ./mvnw -Dmaven.build.cache.enabled=false -pl models/spring-ai-bedrock-converse clean package — Tests run: 87, Failures: 0, Errors: 0, checkstyle and spring-javaformat pass
• Reverted the main-code change and re-ran only the three new tests: the two empty-text tests fail (Tests run: 3, Failures: 2) and the regression guard passes, confirming the tests cover the reported defect

BedrockProxyChatModelIT requires AWS credentials and was not executed.

Note on the all-empty case

A user message with neither text nor media now produces a message with an empty content list. That request was already rejected by the API before this change (as an empty text block), so this is not a regression, and it is out of scope here.