BIP 341: allow taproot_sign_key with no script tree

[jonasnick]

In contrast to taproot_output_script, taproot_sign_key was not able to deal with a script_tree that is None. This commit fixes taproot_sign_key such that it can sign for such outputs.

This commit avoids changing the behavior of the functions except taproot_sign_key at the cost of having some code duplication. Alternatively, one could let taproot_tree_helper deal with a None script_tree directly.

CC @sipa @ajtowns

[Randy808]

Yeah this addresses the issue brought up in #1348 👍

[sipa]

ACK 3b6b398e7723e501939fac993d6ca5ca72f89975

[kallewoof]

Conflicts.

[jonasnick]

rebased