Clef beside header clef saved in 2.x becomes header clef when imported to 3.x

Open a blank score in MuseScore 2, apply a bass clef to the rest in the first measure, keeping the header clef unchanged. Import to MuseScore 3, and that bass clef becomes the header clef.

Relates to #297326: Crash on opening Musescore 2.x file that uses leading space setting on the initial clef and #297328: Importing a 2.x score makes initial clefs lose their customized properties.