Allow for None gradients in GradientAccumulator.
The [TFTrainer commit](https://github.com/huggingface/transformers/commit/aad50151f35b934039c455242c433a24f011cc93#diff-2a765415189cf1edda4414e1758b0e79) seems to have reverted the ability for gradients to be None in GradientAccumulator. This behavior is necessary when pretraining a model, such as AlbertForPreTraining, and using only one loss (only SOP or only MLM).

The [OpenNMT repository](https://github.com/OpenNMT/OpenNMT-tf/blob/master/opennmt/optimizers/utils.py) the code was taken from lacks this ability, but it's important. This commit restores the ability for gradients to be None.