Adjust GraphQL helper to query empty fields

These adjustments make sure our GraphQL helpers support rendering
queries for empty fields like this:

      {
        echo(text: "Hello world")
      }

Instead of like this:

     {
       echo(text: "Hello world") {
       }
     }

The latter would be an invalid query, causing parsing errors.

spec/support/helpers/graphql_helpers.rb

@@ -79,12 +79,21 @@ module GraphqlHelpers
     attributes = attributes_to_graphql(attributes)
     attributes = "(#{attributes})" if attributes.present?
     <<~QUERY
-      #{name}#{attributes} {
-        #{fields}
-      }
+      #{name}#{attributes}
+      #{wrap_fields(fields)}
     QUERY
   end
 
+  def wrap_fields(fields)
+    return unless fields.strip.present?
+
+    <<~FIELDS
+    {
+      #{fields}
+    }
+    FIELDS
+  end
+
   def all_graphql_fields_for(class_name, parent_types = Set.new)
     type = GitlabSchema.types[class_name.to_s]
     return "" unless type
@@ -116,8 +125,8 @@ module GraphqlHelpers
     end.join(", ")
   end
 
-  def post_graphql(query, current_user: nil, variables: nil)
-    post api('/', current_user, version: 'graphql'), params: { query: query, variables: variables }
+  def post_graphql(query, current_user: nil, variables: nil, headers: {})
+    post api('/', current_user, version: 'graphql'), params: { query: query, variables: variables }, headers: headers
   end
 
   def post_graphql_mutation(mutation, current_user: nil)