_top_level_inferred: Handle dist.files being None

Fixes python#344.

Signed-off-by: Anders Kaseorg <andersk@mit.edu>

importlib_metadata/__init__.py

@@ -1023,6 +1023,6 @@ def _top_level_declared(dist):
 def _top_level_inferred(dist):
     return {
         f.parts[0] if len(f.parts) > 1 else f.with_suffix('').name
-        for f in dist.files
+        for f in dist.files or []
         if f.suffix == ".py"
     }