Improve warning about missing schemes

[fredpi]

The case covered with this code is an edge case that would have otherwise been handled a few lines below with the following text:

⚠️ No shared scheme(s) found matching library name 'SwiftEntryKit' – can't remove potentially unnecessary shared schemes, keeping all

As that text is misleading if there a no schemes at all, it's suitable to explicitly handle this case.

[Jeehut]

@fredpi CI is failing, which makes sense: error: 'guard' body must not fall through, consider using a 'return' or 'throw' to exit the scope

Please fix.

[fredpi]

@Dschee My bad, it's not that hard 😂