fix beam with empty note array throwing undefined error in specific percussion samples
fix beam with empty note array throwing undefined error in specific percussion samples

needs further investigation, probably the beams aren't read and displayed correctly here.
sample: test_percussion_display_step_from_xml

opensheetmusicdisplay/opensheetmusicdisplay,opensheetmusicdisplay/opensheetmusicdisplay,opensheetmusicdisplay/opensheetmusicdisplay