fix: could cause infinte recursion, just null is enough

Signed-off-by: Benedek Major <benedek@major.onl>

Author: Bensikrac

app/src/test/java/com/owncloud/android/utils/FilesSyncHelperTest.java

@@ -44,10 +44,7 @@ public void testWalkFileTreeNullHandling(){
         mockPath = mock(Path.class);
         Mockito.when(mockFile.isDirectory()).thenReturn(true);
         Mockito.when(mockPath.toFile()).thenReturn(mockFile);
-        Mockito.when(mockFile.listFiles())
-            .thenReturn(new File[]{mockFile, mockFile, mockFile})
-            .thenReturn(new File[0])
-            .thenReturn(null);
+        Mockito.when(mockFile.listFiles()).thenReturn(null);
         Mockito.when(mockFile.canRead()).thenReturn(true);
 
         SimpleFileVisitor<Path> visitor = new SimpleFileVisitor<Path>() {