UI: Use weak_source_expired instead of getting the source

[gxalpha]

Description

Changes an action that's getting a source from the weak source to check if the source still exists to simply use obs_weak_source_expired.

Motivation and Context

Noticed this, and appears to be unnecessary.

How Has This Been Tested?

macOS 14.4.1
Copied filters from a source.
Right clicked on the audio mixer to see that "Paste Filters" is available.
Deleted the source. Right clicked on the audio mixer again and "Paste Filters" is unavailable.

Types of changes

• Code cleanup (non-breaking change which makes code smaller or more readable)

Checklist:

• My code has been run through clang-format.
• I have read the contributing document.
• My code is not on the master branch.
• The code has been tested.
• All commit messages are properly formatted and commits squashed where appropriate.
• I have included updates to all appropriate documentation.

[RytoEX]

Tested this locally and it seems fine. A second look would be appreciated, but I may not require it.

This code pattern is also used elsewhere.
52497d2