Docs: Clarify get_user_meta() return value for a non-existing meta key on a valid user

[haritpanchal]

Summary

The @return description for get_user_meta() only documents the empty-array/empty-string
default for a non-existing user ID. Tracing through get_metadata_raw() and
get_metadata_default() in wp-includes/meta.php, the same default is returned whenever
get_metadata_raw() resolves to null — which also happens when the user exists but the
given meta key simply has no rows in the meta table. There's no code path that distinguishes
"user doesn't exist" from "user exists, key doesn't."

This clarifies the docblock to cover both cases.

Documentation-Issue-Tracker: WordPress/Documentation-Issue-Tracker#2293

Testing

Docs-only change (PHPDoc comment), no functional code touched. Verified get_metadata_raw()
/ get_metadata_default() behavior by reading wp-includes/meta.php in trunk.

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