refactor(legacy): simplify getTheme and clarify legacy theme docs

[joshtrichards]

Summary

Clean up OC_Util::getTheme() and clarify its PHPDoc.

I found myself needing to make sure I clearly understand the context of this function while doing some other optimization and refactoring work in the URLGenerator.

Details

• simplify control flow with early returns
• rename the local variable for readability
• document that this only applies to legacy filesystem-based themes
• document that configured theme directories are not validated
• clarify the possible return values

No behavior change intended.

Notes

• The default fallback theme doesn't exist in any modern installation so most of the time this is just going to return an empty string (this is why I was looking at this function -- the places we call it from, such as in URLGenenerator::imagePath() tend not to check for an empty return value... leading to unnecessary work in a few spots).
• We can't get rid of this legacy tiny helper outright just yet (though maybe we can/should move somewhere more appropriate and/or rename it).

Checklist

• Code is properly formatted
• Sign-off message is added to all commits
• Tests (unit, integration, api and/or acceptance) are included
• Screenshots before/after for front-end changes
• Documentation (manuals or wiki) has been updated or is not required
• Backports requested where applicable (ex: critical bugfixes)
• Labels added where applicable (ex: bug/enhancement, 3. to review, feature component)
• Milestone added for target branch/version (ex: 32.x for stable32)

AI (if applicable)

• The content of this PR was partly or fully generated using AI