add check is capabilities key is set

[geekofshire]

This PR addresses a fatal error in WP_User_Query that occurs when a user role is missing the capabilities key. This issue is typically caused by third-party plugins that create roles without properly setting capabilities. When such roles exist, calls to array_filter() on null trigger a fatal error.

Trac ticket: https://core.trac.wordpress.org/ticket/62600

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