Fix #19918: Tremolo beam disappearing

[HemantAntony]

Resolves: #19918

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• I created a unit test or vtest to verify the changes I made (if applicable)

[oktophonie]

Not sure what the vtest change here is about, though it doesn't particularly concern me; the issue itself seems to be fixed in this PR.

[HemantAntony]

Oh okay, that sounds great. I was a bit worried about that

[cbjeukendrup]

When I enable Diagnostics > Engraving > Color element shapes, it seems this change seems to have an not-exclusively-positive effect on the calculated shapes:

Before	After