Fix #305724: Disable save prompt for empty scores

[Spire42]

Resolves: #305724

Added logic to disable the save prompt for empty scores; i.e., scores that were automatically created at startup using the “Start with score” feature and that don't contain any modifications.

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[Spire42]

That last force-push was just to clarify a few comments inside MuseScore::closeEvent() (no changes to actual code).