Customize: Prevent PHP notice on theme-switch dashboard welcome link (and link...

Customize: Prevent PHP notice on theme-switch dashboard welcome link (and link to themes admin screen) when user cannot `customize`.

Props skostadinov, ocean90.
Amends [41699].
See #42050.
Fixes #42525.

Built from https://develop.svn.wordpress.org/trunk@42169


git-svn-id: https://core.svn.wordpress.org/trunk@41999 1a063a9b-81f0-0310-95a4-ce76da25c4cd