Editor: Cast the result of the `default_content`, `default_title`, and `default_excerpt` filters.

If a plugin returns a non-string value (or returns `null`) on these filters, it can cause errors in the block editor. Casting them as a string prevents these errors.

Merges [43858] from the 5.0 branch to trunk.

Props dd32.
See #45236.

Built from https://develop.svn.wordpress.org/trunk@44224


git-svn-id: https://core.svn.wordpress.org/trunk@44054 1a063a9b-81f0-0310-95a4-ce76da25c4cd