[flat field] fix attempt to load dark and flats from *tomwer_processes.h5. Xas...

[flat field] fix attempt to load dark and flats from *tomwer_processes.h5. Xas failing if the file was here but didn't contained any dark or flat
3 jobs for fix_tomwer_third_party in 11 minutes and 3 seconds (queued for 3 seconds)