[CI] Ensure teddit urls are not null before filter

Teddit's instance file contains null URL entries, which was breaking the
nightly build. This ensures that the URL exists for an entry before
continuing with processing.

.github/workflows/update-instances.yml

@@ -170,6 +170,7 @@ jobs:
         curl -s https://codeberg.org/teddit/teddit/raw/branch/main/instances.json | \
           jq '[
             .[] |
+            select(.url != null) |
             select(.url | contains("https")) |
             .url ] |
             sort' > teddit-tmp.json