[MusicXML import] prevent font-size=0

MusicXML exporters sometimes generate font-size=0, which is not caught by MuseScore's importer. In 3.x this was caught by the assertion in setFontSize() in libmscore/textbase.h. In master the assert has been removed, but this may still result in an invisible and uneditable text.

For an example see file a.xml, attached to https://musescore.org/en/node/328606.