[mlir][linalg][transform][python] Fix optional args of PadOp mix-in.

The mix-in did not allow to *not* set many of the arguments, even though
they represent optional attributes. Instead, it set default values,
which have different semantics in some cases. In other cases, setting
the default values is already done by the C++ layer, in which case they
are currently redundant and may be wrong in some potential future change
in the TD or C++ files. With this patch, `None` is preserved until the
generated binding, which handles them as desired.

Reviewed By: springerm

Differential Revision: https://reviews.llvm.org/D158844