Don't call `Shape::right()` etc. on empty shapes

See the discussion at https://github.com/musescore/MuseScore/pull/24486.

When calling Shape::right() etc. on empty shapes, ±DBL_MAX is returned, which is a bit dangerous. We could just add if (empty()) { return 0; }, but actually such methods should just not be called on empty shapes. Adding return 0; would hide the problem.

Therefore, we decided that IF_ASSERT_FAILED(!empty()) { return 0; } would be more appropriate. However, this reveals quite a number of cases where we were calling such methods on empty shapes. Some are trivially avoidable, others require a bit more thought. See https://github.com/musescore/MuseScore/compare/master...cbjeukendrup:MuseScore:zzz_backup/shape_right_assert_nonempty for an initial attempt at such fixes.