category with no apps should be ignored, even if defined in config

https://gitlab.com/fdroid/fdroidclient/-/issues/2619#note_1421280589

The test needed to change because the test index files contained category
definitions that were not ever used in the "copy tests/repo, generate java/gpg
keys, update, and gpgsign" test in tests/run-tests.

tests/metadata.TestCase

@@ -1863,6 +1863,8 @@ class MetadataTest(unittest.TestCase):
                 AntiFeatures:
                   - NonFreeNet
                 Categories:
+                  - Multimedia
+                  - Security
                   - Time
                 License: GPL-3.0-only
                 SourceCode: https://github.com/miguelvps/PoliteDroid