[BugFix] Fix DROP FUNCTION IF EXISTS ignoring ifExists flag (backport #69216)

[mergify]

Why I'm doing:

Fixes #69215

What I'm doing:

Fixes #69215

What type of PR is this:

• BugFix
• Feature
• Enhancement
• Refactor
• UT
• Doc
• Tool

Does this PR entail a change in behavior?

• Yes, this PR will result in a change in behavior.
• No, this PR will not result in a change in behavior.

If yes, please specify the type of change:

• Interface/UI changes: syntax, type conversion, expression evaluation, display information
• Parameter changes: default values, similar parameters but with different default values
• Policy changes: use new policy to replace old one, functionality automatically enabled
• Feature removed
• Miscellaneous: upgrade & downgrade compatibility, etc.

Checklist:

• I have added test cases for my bug fix or my new feature
• This pr needs user documentation (for new or modified features or behaviors)
  • I have added documentation for my new feature or new function
  • This pr needs auto generate documentation
• This is a backport pr

Bugfix cherry-pick branch check:

• I have checked the version labels which the pr will be auto-backported to the target branch
  • 4.1
  • 4.0
  • 3.5
  • 3.4

---

This is an automatic backport of pull request #69216 done by [Mergify](https://mergify.com).

[mergify]

🧪 CI Insights

Here's what we observed from your CI run for 5dd3299.

🟢 All jobs passed!

But CI Insights is watching 👀