Fix crash when adding text to a newly created SimpleText element.

When some text is typed for a newly created SimpleText text item (staff, system, ...), the program crashes.

Added a test to check the SimpleTex::_text list is not empty before adding to it.
Fix #24409: crash when adding textual elements

Adds another test for TLine::_text being empty or not.
Merge pull request #666 from mgavioli/Fix_crash_on_adding_to_SimpleText

Fix #24409 : crash when creating a new SimpleText element or adidgin text to an empty one