chenged if statement in DataFrameToCollection function to_list_of_documents

cdplib/db_migration/DataFrameToCollection.py

@@ -90,8 +90,9 @@ class DataFrameToCollection():
 
                 # check that there is only one possible value of this field
                 n_distinct_values = data.groupby(grp_fields, sort=False)[field].nunique().max()
-
-                if n_distinct_values != 1:
+                
+                #n_distinct_valus can be 0 if the column only contains NaN values
+                if n_distinct_values > 1:
                     err = "Field {0} is not unique with respect to {1}"\
                           .format(field, grp_fields)