Fix crash when assets directory doesn't exist

The watchfiles library (Rust notify) crashes when watching a
non-existent directory. Add existence check for app/assets before
yielding it from _iter_asset_dirs().

Author: davegaeddert

plain/plain/assets/finders.py

@@ -54,4 +54,5 @@ def _iter_asset_dirs() -> Iterator[str]:
             yield asset_dir
 
     # The app/assets take priority over everything
-    yield _APP_ASSETS_DIR
+    if _APP_ASSETS_DIR.exists():
+        yield _APP_ASSETS_DIR