Bootstrap/Load: Check if HTTP_HOST is defined when determining the current site.

This avoids a PHP warning when running cron via CLI on Multisite:
{{{
Warning: Undefined array key "HTTP_HOST" in wp-includes/ms-settings.php on line 62
}}}

Follow-up to [https://mu.trac.wordpress.org/changeset/11 mu:11], [https://mu.trac.wordpress.org/changeset/423 mu:423], [https://mu.trac.wordpress.org/changeset/543 mu:543], [12602], [27359].

Props Mte90, JochenT, walterebert, elialum, SergeyBiryukov.
Fixes #47733.

git-svn-id: https://develop.svn.wordpress.org/trunk@60226 602fd350-edb4-49c9-b593-d223f7449a82

Author: SergeyBiryukov

src/wp-includes/ms-settings.php

@@ -59,7 +59,7 @@
 // have not been populated in the global scope through something like `sunrise.php`.
 if ( ! isset( $current_site ) || ! isset( $current_blog ) ) {
 
-	$domain = strtolower( stripslashes( $_SERVER['HTTP_HOST'] ) );
+	$domain = strtolower( stripslashes( $_SERVER['HTTP_HOST'] ?? '' ) );
 	if ( str_ends_with( $domain, ':80' ) ) {
 		$domain               = substr( $domain, 0, -3 );
 		$_SERVER['HTTP_HOST'] = substr( $_SERVER['HTTP_HOST'], 0, -3 );