report: Handle shader stages that don't log a scheduling mode
Without something like this, report.py
bombs out on vec4 platforms with:
Traceback (most recent call last):
File "./report.py", line 387, in <module>
main()
File "./report.py", line 246, in main
get_sched_mode(before[p], after[p]))
BrokenPipeError: [Errno 32] Broken pipe
Exception ignored in: <_io.TextIOWrapper name='<stdout>' mode='w' encoding='UTF-8'>