bug with musicxml2ly

Hi all,

I have uncovered a "bug" in files exported to musicxml from Musescore 3.6.2.

This issue occurs when a single staff is created in the wizard:
1) Select "Treble clef" and create a blank staff.
2) Export to musicxml (compressed, uncompressed---doesn't matter).
Musicxml2ly will fail on this file.

Create the exact same file, but select an instrument in "Choose instrument", and it works perfectly. The troublesome part of the xml file appears to be:

<part-list>
    <part-group type="start" number="1">
      <group-symbol>brace</group-symbol>
      </part-group>

This section is missing in the file that works, and deletion fixes the issue.