warn when a sibling sex has no matching respondents
warn_uninterviewed_sex.RdCalled from aggregate_maternal_estimates when
only_females = FALSE. In most sibling history surveys only women are
interviewed, so there is no respondent age distribution and no visibility
summary for male siblings, and their estimates come out NA. That is the
honest answer – you cannot summarize the visibility of a sex that
was never interviewed – but it should not be silent.