Site Health: Use `WP_Theme::parent()` in the inactive themes test as a more...

Site Health: Use `WP_Theme::parent()` in the inactive themes test as a more reliable check that the parent theme exists.

This is also more consistent with the other instances of directly referencing `WP_Theme::parent()` properties or methods in core.

Props mukesh27, juanlopez4691, davidbaumwald, afragen, SergeyBiryukov.
Fixes #49934.
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git-svn-id: https://core.svn.wordpress.org/trunk@47591 1a063a9b-81f0-0310-95a4-ce76da25c4cd