fix: avoid occurring at least one message error when there was no user message for Claude llm

[dwgeneral]

fix: avoid occurring at least one message error when there was no user message for Claude llm

Checklist:

[!IMPORTANT]
Please review the checklist below before submitting your pull request.

• [x] Please open an issue before creating a PR or link to an existing issue
• [x] I have performed a self-review of my own code
• [x] I have commented my code, particularly in hard-to-understand areas
• [x] I ran dev/reformat(backend) and cd web && npx lint-staged(frontend) to appease the lint gods

Description

The issue related is here

Fixes #8788

Type of Change

• [x] 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
• [ ] Improvement, including but not limited to code refactoring, performance optimization, and UI/UX improvement
• [ ] Dependency upgrade

Testing Instructions

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

• [x] Use Claude model without user message provided to run on workflow

[laipz8200]

Is this plan usable when configuring with a system prompt and a user prompt?

[dwgeneral]

Is this plan usable when configuring with a system prompt and a user prompt?

Yes, I tested the case too, that works fine as well.