General: Ensure duplicated keys existence before validation

Adds array key existence check to prevent undefined index warnings.

src/wp-includes/pluggable.php

@@ -2514,7 +2514,8 @@ function wp_salt( $scheme = 'auth' ) {
 		foreach ( array( 'auth', 'secure_auth', 'logged_in', 'nonce' ) as $key ) {
 			foreach ( array( 'key', 'salt' ) as $second ) {
 				$const = strtoupper( "{$key}_{$second}" );
-				if ( ! defined( $const ) || true === $duplicated_keys[ constant( $const ) ] ) {
+
+				if ( ! defined( $const ) || ( isset( $duplicated_keys[ constant( $const ) ] ) && true === $duplicated_keys[ constant( $const ) ] ) ) {
 					$options_to_prime[] = "{$key}_{$second}";
 				}
 			}