add test for non existing file by id

Signed-off-by: Robin Appelman <robin@icewind.nl>

tests/lib/Files/Cache/CacheTest.php

@@ -553,6 +553,7 @@ class CacheTest extends \Test\TestCase {
 
 	function testNonExisting() {
 		$this->assertFalse($this->cache->get('foo.txt'));
+		$this->assertFalse($this->cache->get(-1));
 		$this->assertEquals(array(), $this->cache->getFolderContents('foo'));
 	}