Fix mypy error

Author: erindru

sqlmesh/core/engine_adapter/bigquery.py

@@ -296,6 +296,8 @@ def get_bq_schema(self, table_name: TableName) -> t.List[bigquery.SchemaField]:
         table = exp.to_table(table_name)
         if len(table.parts) == 3 and "." in table.name:
             self.execute(exp.select("*").from_(table).limit(0))
+            if not self._query_job:
+                raise ValueError("Query job was not created?")
             return self._query_job._query_results.schema
         return self._get_table(table).schema