Customize: Fix logic inversion in [42113] which prevented themes from being...

Customize: Fix logic inversion in [42113] which prevented themes from being installed in Customizer.

Also fix PHP notice related to parent themes and WordPress.org theme query results.

Props dd32, obenland, celloexpressions, westonruter, atachibana for testing.
See #42406, #37661.
Fixes #42442.

Built from https://develop.svn.wordpress.org/trunk@42122


git-svn-id: https://core.svn.wordpress.org/trunk@41953 1a063a9b-81f0-0310-95a4-ce76da25c4cd