Update anonymizer.py

Fixing NA issue: "The problem are the NA entries in your dataset. Each row in your dataset has at least one NA somewhere. When you apply .groupby to NA entries, it wouldn't know how to group NAs so it removes them, leaving an empty result (length 0)." pandas-dev/pandas#23050

Author: jaSunny

anonymizer.py

@@ -783,7 +783,7 @@ def identify_1st_identifier(colname):
     include an error rate for null values etc.
     """
     global statistics
-    representatives = df.groupby(colname, sort=False).size().reset_index().rename(columns={0:'count'})
+    representatives = df.fillna(-1).groupby(colname, sort=False).size().reset_index().rename(columns={0:'count'})
     unique_entries = representatives.loc[representatives['count']==1]['count'].count()
     coverage_of_uniques = unique_entries / ( len(df.index) - df[colname].isnull().sum() )