replace message content type list to string when file_objs is empty ..

[vikeychen]

…in get_history_prompt_messages

Description

When I was testing the chart generation tool with open-source model, I noticed that when there are files involved, although the files are not included in the prompt, the format of the prompt message is {"role":"assistant","content":[{"type":"text",...}]}, whereas the format for regular messages is {"role":"assistant","content":"str"}. my simulated OpenAI API backend does not support the former format. I understand this is an issue on the backend side, but maybe making this change would improve compatibility.

Fixes # (issue)

Type of Change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update, included: Dify Document

How Has This Been Tested?

Please describe the tests that you ran to verify your changes. Provide instructions so we can reproduce. Please also list any relevant details for your test configuration

• TODO

Suggested Checklist:

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• My changes generate no new warnings
• I ran dev/reformat(backend) and cd web && npx lint-staged(frontend) to appease the lint gods
• optional I have made corresponding changes to the documentation
• optional I have added tests that prove my fix is effective or that my feature works
• optional New and existing unit tests pass locally with my changes

[takatost]

LGTM