tests: fix assertions for empty MediaFile.images

used to work due to inconsistent mediafile implementation, but with
beetbox/mediafile#64 (in mediafile >= 0.11.0)
list fields are None if non-existent, not the empty list

test/test_embedart.py

@@ -82,7 +82,7 @@ def test_embed_art_from_file_with_no_input(self):
         self.run_command('embedart', '-f', self.small_artpath)
         mediafile = MediaFile(syspath(item.path))
         # make sure that images array is empty (nothing embedded)
-        self.assertEqual(len(mediafile.images), 0)
+        self.assertFalse(mediafile.images)
 
     def test_embed_art_from_file(self):
         self._setup_data()
@@ -203,7 +203,7 @@ def test_clear_art_with_yes_input(self):
         self.io.addinput('y')
         self.run_command('clearart')
         mediafile = MediaFile(syspath(item.path))
-        self.assertEqual(len(mediafile.images), 0)
+        self.assertFalse(mediafile.images)
 
     def test_clear_art_with_no_input(self):
         self._setup_data()

test/test_zero.py

@@ -103,7 +103,7 @@ def test_album_art(self):
         item.write()
 
         mf = MediaFile(syspath(path))
-        self.assertEqual(0, len(mf.images))
+        self.assertFalse(mf.images)
 
     def test_auto_false(self):
         self.config['zero']['fields'] = ['year']