message: Use a ternary if for checking main_op is None
  Fixes a minor bug in saved_model which raises an
  exception since evaluating ops as booleans are
  forbidden in TF 2.x
  Fixes
diff: @@ -534,7 +534,7 @@ class SavedModelBuilder(_SavedModelBuilder):
  # legacy_init_op is deprecated, and going away in TF 2.0.
  # Re-mapping to main_op, as treatment is identical regardless.
  - main_op = main_op or legacy_init_op
  + main_op = main_op if main_op is not None else legacy_init_op
  # Add assets and ops
  self._add_collections(assets_collection, main_op, None)
  