I18N: In get_user_locale() ensure that $user_id isn't a falsy value before calling get_user_by().

Fixes #38485.
Built from https://develop.svn.wordpress.org/trunk@39134


git-svn-id: http://core.svn.wordpress.org/trunk@39074 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/l10n.php

@@ -92,7 +92,7 @@ function get_user_locale( $user_id = 0 ) {
 		$user = wp_get_current_user();
 	} elseif ( $user_id instanceof WP_User ) {
 		$user = $user_id;
-	} elseif ( is_numeric( $user_id ) ) {
+	} elseif ( $user_id && is_numeric( $user_id ) ) {
 		$user = get_user_by( 'id', $user_id );
 	}