fix: load_table_from_dataframe now assumes there may be local null values

google/cloud/bigquery/_pandas_helpers.py

@@ -178,12 +178,18 @@ def bq_to_arrow_field(bq_field, array_type=None):
     if arrow_type is not None:
         if array_type is not None:
             arrow_type = array_type  # For GEOGRAPHY, at least initially
-        is_nullable = bq_field.mode.upper() == "NULLABLE"
         metadata = BQ_FIELD_TYPE_TO_ARROW_FIELD_METADATA.get(
             bq_field.field_type.upper() if bq_field.field_type else ""
         )
         return pyarrow.field(
-            bq_field.name, arrow_type, nullable=is_nullable, metadata=metadata
+            bq_field.name,
+            arrow_type,
+            # Even if the remote schema is REQUIRED, there's a chance there's
+            # local NULL values. Arrow will gladly interpret these NULL values
+            # as non-NULL and give you an arbitrary value. See:
+            # https://github.com/googleapis/python-bigquery/issues/1692
+            nullable=True,
+            metadata=metadata,
         )
 
     warnings.warn("Unable to determine type for field '{}'.".format(bq_field.name))