fix: add default value in SchemaField.from_api_repr() (#2115)

Author: Linchin

google/cloud/bigquery/schema.py

@@ -228,6 +228,12 @@ def from_api_repr(cls, api_repr: dict) -> "SchemaField":
         # fields. See https://github.com/googleapis/python-bigquery/issues/6
         placeholder._properties = api_repr
 
+        # Add the field `mode` with default value if it does not exist. Fixes
+        # an incompatibility issue with pandas-gbq:
+        # https://github.com/googleapis/python-bigquery-pandas/issues/854
+        if "mode" not in placeholder._properties:
+            placeholder._properties["mode"] = "NULLABLE"
+
         return placeholder
 
     @property