Skip None paths when looking for modules

Change-Id: I49fb38b56f868a730b622e20cf8d5781f13fe5e3
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/30580
Commit-Queue: dan sinclair <dsinclair@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>

generator/generator_lib.py

@@ -201,6 +201,10 @@ def _compute_python_dependencies(root_dir=None):
 
     paths = set()
     for path in module_paths:
+        # Builtin/namespaced modules may return None for the file path.
+        if not path:
+            continue
+
         path = os.path.abspath(path)
 
         if not path.startswith(root_dir):