Clarify emptyIncludeConf logic

Summary:
Suggestion from John Newbery <john@johnnewbery.com> in
https://github.com/bitcoin/bitcoin/pull/15934#discussion_r343795528

This is a partial backport of Core PR15934 : https://github.com/bitcoin/bitcoin/pull/15934/commits/dc8e1e75487461ec9bff433144f0db831b682403

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D5884