Add is_nullable method to check for NULLABLE mode

Resolves: #3548

Author: Son CHU

bigquery/google/cloud/bigquery/dbapi/cursor.py

@@ -76,7 +76,7 @@ def _set_description(self, schema):
                 internal_size=None,
                 precision=None,
                 scale=None,
-                null_ok=field.mode == 'NULLABLE')
+                null_ok=field.is_nullable)
             for field in schema])
 
     def _set_rowcount(self, query_results):

bigquery/google/cloud/bigquery/schema.py

@@ -65,6 +65,11 @@ def mode(self):
         """
         return self._mode
 
+    @property
+    def is_nullable(self):
+        """Check whether 'mode' is 'nullable'."""
+        return self._mode == 'NULLABLE'
+
     @property
     def description(self):
         """Optional[str]: Description for the field."""