Database: Check if the $args[0] value exists in wpdb::prepare() before accessing it.

This avoids an `Undefined array key 0` PHP warning if no values are passed to the method besides the query string.

Follow-up to [41470].

Props mjaschen.
Fixes #54453.

git-svn-id: https://develop.svn.wordpress.org/trunk@52206 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/wp-db.php

@@ -1395,7 +1395,7 @@ class wpdb {
 
 		// If args were passed as an array (as in vsprintf), move them up.
 		$passed_as_array = false;
-		if ( is_array( $args[0] ) && count( $args ) === 1 ) {
+		if ( isset( $args[0] ) && is_array( $args[0] ) && 1 === count( $args ) ) {
 			$passed_as_array = true;
 			$args            = $args[0];
 		}