message: Teach XlaTensorBuffer to not deallocate null pointers
  Zero sized XlaTensorBuffer can have null as the backing storage.  De-allocating
  a null pointer produces an annoying warning from the BFCAllocator.
diff: @@ -122,7 +122,11 @@ class XlaTensorBuffer : public TensorBuffer {
  data_ = const_cast<void*>(ptr);
  }
  - ~XlaTensorBuffer() override { allocator_->DeallocateRaw(data_); }
  + ~XlaTensorBuffer() override {
  + if (data_) {
  + allocator_->DeallocateRaw(data_);
  + }
  + }
  void* data() const override { return data_; }
  size_t size() const override { return expected_size_; }
  