General: Correct force_ssl_content() to always return a boolean value.

This aims to bring parity with `force_ssl_admin()`.

Includes:
* Allowing `force_ssl_content()` to properly accept `false` as a value.
* Correcting an erroneous `! $force` conditional that should have been reversed.
* Adding unit tests to confirm valid behavior.

Follow-up to [https://mu.trac.wordpress.org/changeset/1979 mu:1979], [11903], [12603], [47808], [59830].

Props justlevine for initial patch.
See #52217.
Built from https://develop.svn.wordpress.org/trunk@60147


git-svn-id: http://core.svn.wordpress.org/trunk@59483 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/ms-functions.php

@@ -2420,15 +2420,15 @@ Thanks!
  *
  * @since 2.8.5
  *
- * @param bool $force
+ * @param bool|null $force Optional. Whether to force SSL in admin screens. Default null.
  * @return bool True if forced, false if not forced.
  */
-function force_ssl_content( $force = '' ) {
+function force_ssl_content( $force = null ) {
 	static $forced_content = false;
 
-	if ( ! $force ) {
+	if ( ! is_null( $force ) ) {
 		$old_forced     = $forced_content;
-		$forced_content = $force;
+		$forced_content = (bool) $force;
 		return $old_forced;
 	}
 

wp-includes/version.php

@@ -16,7 +16,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '6.9-alpha-60142';
+$wp_version = '6.9-alpha-60147';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.