allow diagnosisType to be cleared out from patient records

The unusual PUT/POST semantics of clinic's patient update endpoint
strike again.

BACK-3872

Author: ewollesen

reference/metadata/models/diagnosisType.v1.yaml

@@ -10,4 +10,5 @@ enum:
   - mody
   - other
   - notApplicable
+  - "" # used to clear out an existing value
 example: 'type1'