fix presubmit

Author: shuoweil

bigframes/series.py

@@ -609,6 +609,15 @@ def astype(
         if errors not in ["raise", "null"]:
             raise ValueError("Argument 'errors' must be one of 'raise' or 'null'")
         dtype = bigframes.dtypes.bigframes_type(dtype)
+
+        # BigQuery doesn't support CAST(json_col AS STRING), but it does support
+        # TO_JSON_STRING(json_col).
+        if (
+            self.dtype == bigframes.dtypes.JSON_DTYPE
+            and dtype == bigframes.dtypes.STRING_DTYPE
+        ):
+            return self._apply_unary_op(ops.json_ops.ToJSONString())
+
         return self._apply_unary_op(
             bigframes.operations.AsTypeOp(to_type=dtype, safe=(errors == "null"))
         )