Auto merge of #1579 - nical:update-resources-empty, r=kvark
Auto merge of #1579 - nical:update-resources-empty, r=kvark

Avoid Sending an empty message if there is no resource to update.

I tiny detail, but it is more convenient to check here and always call `update_resources` in gecko/servo than check whether the update list isn't empty before calling `update_resources`. Typically (in particular in gecko), the `ResourceUpdates` is passed around and the code that adds updates is different from the one that sends the transaction.

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