We'd rather aggressively clear asset lists on file save since 9a9e2e19a9 (and
some related commits), seems like we can be a bit more precise here. Only
reload assets from the current file when saving the file, which is quite fast
to do and doesn't happen asynchronously (so UIs showing assets will only show
up once the assets are ready).
Actually we might not have to clear on file save at all, but needs more
testing.
Also fixes disappearing of asset previews on file save.
Pull Request: https://projects.blender.org/blender/blender/pulls/137117