Ensure WP_Theme::$template is not null

[IanDelMar]

Trac ticket: https://core.trac.wordpress.org/ticket/64582

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[github-actions]

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[westonruter]

My first thought was that the WP_Theme::$template should be updated from ?string to string here with this change:

wordpress-develop/src/wp-includes/class-wp-theme.php

Lines 158 to 167 in 905ce8f

	/**
	* The directory name of the theme's files, inside the theme root.
	*
	* In the case of a child theme, this is the directory name of the parent theme.
	* Otherwise, 'template' is the same as 'stylesheet'.
	*
	* @since 3.4.0
	* @var ?string
	*/
	private $template;

But I see that it should indeed be left unset/null in the case of an error, where previously it short-circuits:

wordpress-develop/src/wp-includes/class-wp-theme.php

Lines 287 to 295 in 905ce8f

	if ( is_array( $cache ) ) {
	foreach ( array( 'block_template_folders', 'block_theme', 'errors', 'headers', 'template' ) as $key ) {
	if ( isset( $cache[ $key ] ) ) {
	$this->$key = $cache[ $key ];
	}
	}
	if ( $this->errors ) {
	return;
	}

But I see you addressed this in the ticket description 👍🏻

[westonruter]

In cdb05c8 I added tests to cover the issues you covered in the "Resulting behaviour" section of the ticket.

Without the fixes you've added, there are test failures:

There were 2 failures:

1) Tests_Theme_wpTheme::test_child_theme_with_itself_as_parent_should_have_template_set
The template was not set to the stylesheet.
Failed asserting that null is identical to 'child-parent-itself'.

/var/www/tests/phpunit/tests/theme/wpTheme.php:202

2) Tests_Theme_wpTheme::test_child_theme_with_itself_as_parent_should_have_template_set_when_read_from_cache
The template was not restored from the cache.
Failed asserting that null is identical to 'child-parent-itself'.

/var/www/tests/phpunit/tests/theme/wpTheme.php:232

FAILURES!
Tests: 629, Assertions: 7358, Failures: 2, Skipped: 15.

And with your changes, they are fixed.

[github-actions]

A commit was made that fixes the Trac ticket referenced in the description of this pull request.

SVN changeset: 63507
GitHub commit: 696f277

This PR will be closed, but please confirm the accuracy of this and reopen if there is more work to be done.