[mlir][tensor][bufferize] `tensor.empty` bufferizes to allocation (#68201)

`BufferizableOpInterface::bufferizesToAllocation` is queried when
forming equivalence sets during bufferization. It is not really needed
for ops like `tensor.empty` which do not have tensor operands, but it
should be added for consistency.

This change should have been part of #68080. No test is added because
the return value of this function is irrelevant for ops without tensor
operands. (However, this function acts as a form documentation,
describing the bufferization semantics of the op.)