Filesystem API: Avoid a PHP notice in `WP_Filesystem_Direct::owner()` and...

Filesystem API: Avoid a PHP notice in `WP_Filesystem_Direct::owner()` and `::group()` methods and their `WP_Filesystem_SSH2` counterparts.

Although not officially documented in the PHP manual, `posix_getpwuid()` and `posix_getgrgid()` can return `false` in some circumstances.

Props logig.
Fixes #50373.
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