Allow the header and footer to be unset in rbt review edit.
Review Request #12188 — Created March 21, 2022 and submitted
rbt review editallowed the header and footer to be set, but they then
couldn't be unset. This was due to checking for truthiness when
determining when to set a new value.
Now it checks for presence in
optionsinstead. This allows callers to
--header ""to unset a header.
Tested setting and then unsetting headers and footers on a review.