Fix Merge Request analytics query optimization

This change fixes the MR analytics query optimization. The optimization
was not triggered because additional arguments with null values were
passed by the frontend.

app/graphql/resolvers/project_merge_requests_resolver.rb

@@ -22,7 +22,7 @@ module Resolvers
     def only_count_is_selected_with_merged_at_filter?(args)
       return unless lookahead
 
-      argument_names = args.except(:lookahead, :sort, :merged_before, :merged_after).keys
+      argument_names = args.compact.except(:lookahead, :sort, :merged_before, :merged_after).keys
 
       # no extra filtering arguments are provided
       return unless argument_names.empty?
@@ -34,7 +34,7 @@ module Resolvers
       #   totalTimeToMerge
       # }
       allowed_selected_fields = [:count, :total_time_to_merge]
-      selected_fields = lookahead.selections.map(&:field).map(&:original_name)
+      selected_fields = lookahead.selections.map(&:field).map(&:original_name) - [:__typename] # ignore __typename meta field
 
       # only the allowed_selected_fields are present
       (selected_fields - allowed_selected_fields).empty?

spec/requests/api/graphql/project/merge_requests_spec.rb

@@ -407,10 +407,12 @@ RSpec.describe 'getting merge request listings nested in a project' do
       end
 
       let(:query) do
+        # Note: __typename meta field is always requested by the FE
         graphql_query_for(:project, { full_path: project.full_path },
         <<~QUERY
-        mergeRequests(mergedAfter: "2020-01-01", mergedBefore: "2020-01-05", first: 0) {
+        mergeRequests(mergedAfter: "2020-01-01", mergedBefore: "2020-01-05", first: 0, sourceBranches: null, labels: null) {
           count
+          __typename
         }
         QUERY
         )