Avoid nonce_function_rfc6979 algo16 argument emulation.

Avoid nonce_function_rfc6979 algo16 argument emulation.

This avoids data=NULL and data = zeros to producing the same nonce.

Previously the code tried to avoid the case where some data inputs
 aliased algo16 inputs by always padding out the data.

But because algo16 and data are different lengths they cannot
 emulate each other, and the padding would match a data value of
 all zeros.