Merge #10783: [RPC] Various rpc argument fixes

Summary:
4dc1915 check for null values in rpc args and handle appropriately (Gregory Sanders)
999ef20 importmulti options are optional (Gregory Sanders)
a70d025 fixup some rpc param counting for rpc help (Gregory Sanders)

Pull request description:

Audited where named args will fail to use correct default values or may fail when additional optional arguments are added.

Previously for these parameters, it was fine to omit them as positional arguments, but it would trigger UniValue runtime errors to set them to null, or to omit them while passing named parameters with greater positions (which would internally set earlier missing arguments to null). Now null values are treated the same as missing values so these errors do not occur.

Included a few other small fixes while working on it.

I didn't bother fixing account-based rpc calls.

Tree-SHA512: 8baf781a35bd48de7878d4726850a580dab80323d3416c1c146b4fa9062f8a233c03f37e8ae3f3159e9d04a8f39c326627ca64c14e1cb7ce72538f934ab2ae1e

Backport of Core PR 10783
https://github.com/bitcoin/bitcoin/pull/10783/files
Completes T552

Test Plan:
make check
test_runner.py

Reviewers: deadalnix, Fabien, #bitcoin_abc

Reviewed By: Fabien, #bitcoin_abc

Subscribers: teamcity, schancel

Differential Revision: https://reviews.bitcoinabc.org/D2688