fix: improve content check and streamline content list generation in vlm_middle_json_mkcontent.py

[myhloli]

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Pull request overview

This PR improves content validation and refactors the content list generation logic in the VLM middle JSON content maker. The changes focus on safer dictionary access patterns and code organization.

• Improved content checking to use safe dictionary access with .get() and filter whitespace-only content
• Refactored CONTENT_LIST_V2 generation flow by eliminating early continue and restructuring the conditional logic

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