Fix error in static analysis build.

tests/bootstrap.php

@@ -136,5 +136,5 @@ ini_set('session.gc_divisor', '1');
 session_id('cli');
 
 // Create test database schema
-$schema = new SchemaGenerator(env('FIXTURE_SCHEMA_METADATA'), 'test');
+$schema = new SchemaGenerator(env('FIXTURE_SCHEMA_METADATA', ''), 'test');
 $schema->reload();