Fix IES lights asset tracing

Fix an issue where BAT failed with a `KeyError` (because it couldn't
find the `filepath` property) when handling IES light files.

Also add example files & unit tests.

Reviewed-on: https://projects.blender.org/blender/blender-asset-tracer/pulls/92886

blender_asset_tracer/trace/blocks2assets.py

@@ -244,7 +244,7 @@ def lamp(block: blendfile.BlendFileBlock) -> typing.Iterator[result.BlockUsage]:
         if path is None:
             continue
 
-        yield result.BlockUsage(block, path, path_full_field=field)
+        yield result.BlockUsage(storage, path, path_full_field=field)
 
 
 @dna_code("VO")