UI: Return std::optional from ParseThemeMeta

[derrod]

Description

Use std::optional as a return value instead of a pointer, thus avoiding any manual memory management shenanigans.

Motivation and Context

Want to avoid errors like the one fixed by #10838

This function could probably throw an exception to signal the error, but it currently doesn't and I don't want to make too many changes.

How Has This Been Tested?

Verified themes still work.

Types of changes

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

Checklist:

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