fix(custom_tool): omit optional parameters instead of setting them to None

[ACAne0320]

Summary

fixed #18976

Checklist

• This change requires a documentation update, included: Dify Document
• I understand that this PR may be closed in case there was no previous discussion or issues. (This doesn't apply to typos!)
• I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
• I've updated the documentation accordingly.
• I ran dev/reformat(backend) and cd web && npx lint-staged(frontend) to appease the lint gods

[Copilot]

Pull Request Overview

This PR updates the HTTP request builder to omit optional parameters when they aren’t provided, rather than including them with a null value.

• Remove assignment of None for missing optional parameters
• Add a no-op (pass) with explanation in the parameter-handling branch

Comments suppressed due to low confidence (1)

api/core/tools/custom_tool/tool.py:215

• Update the docstring for do_http_request to mention that optional parameters are now omitted instead of being set to None, so consumers know this change in behavior.

                        else: