Merge pull request #3639 from eaidova/ea/fix_paths_is_none

omz converter fix for case when no paths

Author: eaidova

tools/model_tools/src/openvino/model_zoo/internal_scripts/pytorch_to_onnx.py

@@ -46,7 +46,8 @@ def __init__(self, paths):
 
     def __enter__(self):
         self._original_path = copy.deepcopy(sys.path)
-        sys.path = self._preprended_paths + sys.path
+        if self._preprended_paths is not None:
+            sys.path = self._preprended_paths + sys.path
 
     def __exit__(self, type, value, traceback):
         if self._original_path is not None: