Check if realpath() returns false

realpath() returns false in case the directory does not exist. Found it while preparing strict_typing for PHP7+.

Signed-off-by: Morris Jobke <hey@morrisjobke.de>

lib/private/DB/MigrationService.php

@@ -186,7 +186,7 @@ class MigrationService {
 
 	protected function findMigrations() {
 		$directory = realpath($this->migrationsPath);
-		if (!file_exists($directory) || !is_dir($directory)) {
+		if ($directory === false || !file_exists($directory) || !is_dir($directory)) {
 			return [];
 		}