fix: preserve literal 'NULL' strings for primary key columns

pipeline/plugins/extractors/tap-uk-ofsted/tap_uk_ofsted/tap.py

@@ -237,18 +237,15 @@ class OfstedInspectionsStream(Stream):
 
         for _, row in df.iterrows():
             record = {}
-            for key in self.schema["properties"].keys():
-                record[key] = None
-
             for field, col in col_map.items():
                 val = row.get(col, None)
-                if val == 'NULL':
+                if pd.isna(val):
                     val = None
                 record[field] = val
 
             # Cast URN
             try:
-                record["urn"] = int(record["urn"])
+                record["urn"] = int(record.get("urn"))
             except (ValueError, KeyError, TypeError):
                 continue