Return false instead of crashing in Tensor::SharesBufferWith if neither tensor...

Return false instead of crashing in Tensor::SharesBufferWith if neither tensor has a buffer assigned yet, since that is a valid state. Returning

  buf_ != nullptr && b.buf_ != nullptr &&  buf_->root_buffer() == b.buf_->root_buffer();

still satisfies the contract in the header, i.e. "True iff the two tensors use the same underlying refcounted storage."

PiperOrigin-RevId: 185431574