PHP 7.4: Fix another instance of array access on a datatype that cann…

…ot be accessed as an array.

PHP 7.4 adds a warning when trying access a null/bool/int/float/resource (everything but array, string and object) as if it were an array.

Follow up of [45639].

Props desrosj, jrf.
See #47704.

git-svn-id: https://develop.svn.wordpress.org/trunk@46185 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-admin/includes/update.php

@@ -707,7 +707,7 @@ function maintenance_nag() {
 		 * This flag is cleared whenever a successful update occurs using Core_Upgrader.
 		 */
 		$comparison = ! empty( $failed['critical'] ) ? '>=' : '>';
-		if ( version_compare( $failed['attempted'], $wp_version, $comparison ) ) {
+		if ( isset( $failed['attempted'] ) && version_compare( $failed['attempted'], $wp_version, $comparison ) ) {
 			$nag = true;
 		}
 	}