local: fix getXattr returning empty map instead of nil

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At least on my macOS Sequoia 15.7.4, the system automatically adds a
com.apple.provenance xattr to files created by processes. This xattr
lacks the "user." prefix so getXattr filters it out, but the metadata
map was already allocated, resulting in an empty non-nil map being
returned instead of nil.

This caused TestMetadata/Symlink/Xattr and TestMetadata/File/Xattr
to fail because they assert the return value is nil when no user
xattrs are present.

The fix checks if the metadata map is empty after filtering and
returns nil if so.

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