Fix #306807: Crash on [Go to first empty trailing measure] command with specific circumstance

[worldwideweary]

Resolves: https://musescore.org/en/node/306807

New navigation commands were implemented into 3.x (#5620 for Master -> manually added into 3.5). After 3.5Beta release, I've run into one problem that I didn't catch earlier: If issuing "Go To First Empty Trailing Measure" when there is no selection at all whatsoever + there is no first empty trailing measure, then a crash occurs! Obviously that needed to be fixed, so this PR makes sure that under that circumstance, instead of crashing, and instead of doing nothing, the result is to take the user to the last measure of the score. With this change, I had to also be consistent with the use case when using a specific staff with the same problem, i.e., no empty trailing measure. Before, nothing would happen and then the issuing of the command would be exactly equivalent to entering into Note Entry at current location. Now, this is congruous with no selection at all: take the user to the last measure, only retain the current staff in this instance.

Now I can not get this to crash at all, and in a sense there's some added functionality in that this doubles-down in allowing the user to go to the end of the score if the score is already full or has no empty trailing measure rather than doing nothing.

P.S. I will need to update accordingly the open PR mentioned at the top of this description for the master branch which has yet to be merged.

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