[mlir][Tensor] Allow builders of `tensor.empty` to accept encoding attribute.

The `RankedTensorType` can have an optional encoding
attribute. Allowing the builders of `tensor.empty` to accept the
encoding attribute (optionally), allows building empty tensors with
the type having the encoding attribute.

Reviewed By: nicolasvasilache, hanchung, springerm

Differential Revision: https://reviews.llvm.org/D137297