Fix an error when a user tries to search nonexistent remote user (regression from #4275) (#4400)

app/services/account_search_service.rb

@@ -18,7 +18,7 @@ class AccountSearchService < BaseService
     return [] if query_blank_or_hashtag? || limit < 1
 
     if resolving_non_matching_remote_account?
-      [ResolveRemoteAccountService.new.call("#{query_username}@#{query_domain}")]
+      [ResolveRemoteAccountService.new.call("#{query_username}@#{query_domain}")].compact
     else
       search_results_and_exact_match.compact.uniq.slice(0, limit)
     end