fix #20989: Crash if exporting empty score
Fix #20614 - Parts created from TAB staves have pitched staff.

Staff type was not set in createExcerpt() (libmscore/excerpt.cpp)
Merge pull request #335 from mgavioli/Fix_20614_Parts_from_tabs_have_pitched_staff

Fix #20614 - Parts created from TAB staves have pitched staff.