Cast the result of the `default_content`, `default_title`, and `default_excerpt` filters

[dd32]

WordPress expects that the result of the default_* post filters are strings, but PHP isn't very strict about what an empty string is.

Gutenberg has run into an issue when the filters return null: ​https://github.com/WordPress/gutenberg/issues/11176

It seems reasonable to expect that the related values within the default $post object are strings (ref of where it's used)

Attached patch does just this, casting to a string to allow Gutenberg to receive the empty string that it expects in this case.

[danielbachhuber]

Looks good to me.

[pento]

In 43858:

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.

Props dd32.
See #45236.

[jeremyfelt]

In 44224:

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.