Handle missing tensorflowjs package in unit tests (#1647)

Signed-off-by: Tom Wildenhain <[email protected]>

Author: TomWildenhain-Microsoft

tests/backend_test_base.py

@@ -198,7 +198,10 @@ def freeze_and_run_tf(self, func, feed_dict, outputs, as_session, premade_placeh
         return result, graph_def, initialized_tables
 
     def convert_to_tfjs(self, graph_def_path, output_names):
-        from tensorflowjs.converters import converter
+        try:
+            from tensorflowjs.converters import converter
+        except ImportError:
+            return None
         tfjs_path = os.path.join(self.test_data_directory, self._testMethodName + "_tfjs")
         try:
             converter.convert([graph_def_path, tfjs_path, '--input_format', 'tf_frozen_model',