[backport#15931 5/9] Refactor some importprunedfunds checks with guard clause
Summary:
Credit to jkczyz
https://github.com/bitcoin/bitcoin/pull/15931/commits/769ff05e48fb53d4b62c59060424a0fea71d0aab
Depends on D7509
Partial backport of Core PR15931
Test Plan:
ninja check check-functional
Reviewers: #bitcoin_abc, Fabien
Reviewed By: #bitcoin_abc, Fabien
Differential Revision: https://reviews.bitcoinabc.org/D7510