Fix crash in elasticsearch_check.rb (mastodon#21006)

Nil unwrap causes the admin dashboard to crash/500 when the Chewy client info version number value is nil.
This occurs when running another ES-compatible backend such as MeiliSearch.
Obviously it would be good for chewy to recognise upstream but at least avoiding the crash would be fine.

app/lib/admin/system_check/elasticsearch_check.rb

@@ -34,6 +34,7 @@ def required_version
   end
 
   def compatible_version?
+    return false if running_version.nil?
     Gem::Version.new(running_version) >= Gem::Version.new(required_version)
   end
 end