Fix feature flag check failing when database is empty

As seen in QA pipelines and discussed in
https://gitlab.com/gitlab-org/gitlab/-/merge_requests/42229, the Grape
API initialization would fail if the database were empty. We now check
the existence of the `features` table to prevent this from happening.

This still does not allow us to change the feature flag dynamically, but
it unblocks the test failures.

lib/api/api.rb

@@ -118,7 +118,7 @@ module API
 
     # There is a small chance some users depend on the old behavior.
     # We this change under a feature flag to see if affects GitLab.com users.
-    if Feature.enabled?(:api_json_content_type)
+    if Gitlab::Database.cached_table_exists?('features') && Feature.enabled?(:api_json_content_type)
       content_type :json, 'application/json'
     else
       content_type :txt, 'text/plain'