Prevent empty text on Tempo Text
Discovered in https://musescore.org/en/node/203101
1. create a new tempo marking
2. go in text edit mode on the marking
3. delete all text
Result: Tempo marking is still present, but there is no visible way for the user to know.
Expected Result: either prevent this and make the marking invisible or (from a consistency pov with other text elements) remove the marking if its text is cleared.