Fix a NestedLoader code path that failed to use its meta transform

[greeble-dev]

Solution

While poking around NestedLoader I noticed that one code path fails to pass on the loader's meta transform.

- .get_or_create_path_handle(path, None)
+ .get_or_create_path_handle(path, self.meta_transform)

This seems like a simple oversight - all other code paths pass on the meta transform where possible - although I'm not familiar enough with the asset system to be 100% sure. It was introduced when asset loaders were changed to use the builder pattern (#13465).

Unfortunately I couldn't find an example that actually hits this code path. So I don't have a good test case, and I don't know if any users have experienced it as a bug.

Testing

cargo test -p bevy_asset

Also tested various examples - testbed_2d, testbed_3d and everything in examples/asset.