Util: Remove redundant calls to gArgs.IsArgSet()

Summary:
Return empty std::vector<std::string> with ArgsManager::GetArgs if
nothing is set for that string

This is a backport of core PR10118

Test Plan:

make check

Reviewers: #bitcoin_abc, schancel

Reviewed By: #bitcoin_abc, schancel

Subscribers: teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1781