Number of patient related studies is invalid when PatientID is empty

[amazy]

When displaying the number of patient related studies, since the search is performed against "PatientID": "" all studies are actually matching.

Note, however, that, when modifying the patient tags, only the patients whose PatientIDs is empty are modified.

Reference: https://discourse.orthanc-server.org/t/orthanc-explorer-2-modify-tags-bug/5708/5