Fix resolving loader when Loader.for is used without load (#31)

Author: dylanahsmith

lib/graphql/batch/loader.rb

@@ -33,7 +33,8 @@ def load_many(keys)
     end
 
     def resolve #:nodoc:
-      load_keys = @queue
+      load_keys = queue
+      return if load_keys.empty?
       @queue = nil
       perform(load_keys)
       check_for_broken_promises(load_keys)

test/loader_test.rb

@@ -145,4 +145,9 @@ def test_load_on_different_loaders
   def test_derived_cache_key
     assert_equal [:a, :b, :a], DerivedCacheKeyLoader.load_many([:a, :b, "a"]).sync
   end
+
+  def test_loader_for_without_load
+    loader = EchoLoader.for
+    GraphQL::Batch::Executor.current.wait_all
+  end
 end