FIX: case of empty property values

Summary

This error seems to indicate that the case where the property value is None is not treated properly

Test Environment

pipeline

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src/caosdb/common/datatype.py

@@ -90,6 +90,7 @@ def is_reference(datatype):
     else:
         return True
 
+
 def get_referenced_recordtype(datatype):
     """Return the record type of the referenced datatype.
 
@@ -119,7 +120,8 @@ def get_referenced_recordtype(datatype):
             raise ValueError("list does not have a list datatype")
 
     if datatype == FILE:
-        raise ValueError("FILE references are not considered references with a record type")
+        raise ValueError(
+            "FILE references are not considered references with a record type")
 
     return datatype