Potential crash when having last system with only one measure and then performing operation that causes that measure to move to the previous system

Issue type

Crash or freeze

Description with steps to reproduce

It's as the title says; an easy instance is the following:

Create 1-instrument empty score with standard settings; note that there are three systems
Add a system break between the pre-last and last measure, so that a new fourth system containing only that last measure is constructed
Remove that system break, so that the last measure moves back to the third system

Now, the fourth system contains no measures anymore, so it should be deleted. But that is apparently not what happens; the layout code doesn't handle this case correctly.

In Address Sanitizer builds, a crash occurs, because systems.back()->measures() is empty but we call .back() on it: https://github.com/musescore/MuseScore/blob/b86e6421b3155c4841129336bd624552d1ea0f00/src/engraving/rendering/score/pagelayout.cpp#L84 In release builds, most likely no crash will occur, but .back() reads potentially garbage data, which could cause problems. So, although it doesn't cause a crash immediately, it would be good to fix.

Supporting files, videos and screenshots

https://github.com/user-attachments/assets/9738bdf0-96b7-4879-8067-61a78f4d439f

What is the latest version of MuseScore Studio where this issue is present?

4.5 (master)

Regression

I was unable to check

Operating system

Additional context

No response

Checklist

[X] This report follows the guidelines for reporting bugs and issues
[X] I have verified that this issue has not been logged before, by searching the issue tracker for similar issues
[X] I have attached all requested files and information to this report
[X] I have attempted to identify the root problem as concisely as possible, and have used minimal reproducible examples where possible

musescore / MuseScore