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.
Built from https://develop.svn.wordpress.org/trunk@52206


git-svn-id: http://core.svn.wordpress.org/trunk@51798 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/version.php

@@ -16,7 +16,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '5.9-alpha-52205';
+$wp_version = '5.9-alpha-52206';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.

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];
 		}