Add checks that CSS endpoint response bodies are nonempty
Add checks that CSS endpoint response bodies are nonempty

Change-Id: I0eef335699c8d0d136d6e0d2a288f99eadb95911

wikimedia/mediawiki-services-mobileapps,wikimedia/mediawiki-services-mobileapps,wikimedia/mediawiki-services-mobileapps,wikimedia/mediawiki-services-mobileapps,wikimedia/mediawiki-services-mobileapps,wikimedia/mediawiki-services-mobileapps