Store ::nil markers in CachedMetadataProvider for failed lookups (#47589)

* Fix typo in MetadataProvider docstring

* Fix typo in caching-test: s/Second/Third/

* Store ::nil markers in CachedMetadataProvider for failed lookups

Store ::nil markers in CachedMetadataProvider for any ids for which the wrapped/uncached upstream provider fails to return
metadatas. This prevents repeatedly querying the uncached-provider for ids that don't exist.

The downside is that if the uncached-provider suddenly starts returning metadata for an id that previously did not
exist, we won't pick up on it, but the assumption here is that this is no different / worse than cache invalidation
for existing ids that happen to change after we cache them.

* PR suggestion: remove unnecessary test assertion