Fixed last_executed_query test failure on Oracle

django/db/backends/oracle/base.py

@@ -269,7 +269,7 @@ WHEN (new.%(col_name)s IS NULL)
         # http://cx-oracle.sourceforge.net/html/cursor.html#Cursor.statement
         # The DB API definition does not define this attribute.
         statement = cursor.statement
-        if not six.PY3 and not isinstance(statement, unicode):
+        if statement and not six.PY3 and not isinstance(statement, unicode):
             statement = statement.decode('utf-8')
         # Unlike Psycopg's `query` and MySQLdb`'s `_last_executed`, CxOracle's
         # `statement` doesn't contain the query parameters. refs #20010.