Avoid allocations when dataloader has no work

[tonyghita]

Our GraphQL server has many dataloaders registered. I noticed that each dataloader is dispatched, and lists are allocated even when the dataloader has no work to do.

https://github.com/graphql-java/java-dataloader/blob/master/src/main/java/org/dataloader/DataLoaderHelper.java#L163-L169

It would be great to avoid allocating these lists when the dataloader doesn't have work to do, especially when GraphQL queries are large (many calls to dispatchAll(), and there are many dataloaders registered that are sparsely used.