[FIX] address dm_qc_report edge case + db update issues

[DESm1th]

This PR addresses three bugs in dm_qc_report.py

• Phantoms with anat data were resubmitting to the queue every night, because their manifest file contained only an empty dict, leading to the file write being skipped. This led these phantoms to appear as though they had not completely generated. (2861e75)
• Scan lengths were not being added to the database because a relative path was being given to update_dashboard and the files couldnt be found. The error was also being missed in the logs, because the log level was debug. (0e195e3)
• Header diffs werent updating when a new gold standard is found, because the new standard was not being provided to the function call. This led to some sessions appearing as though they had not fully generated and repeatedly resubmitting to the queue. (8c8317d)