b*starmix() terms should autodetect when focus node does not have nonmissing values.

[krivit]

I.e., if we want homophily among the alter nodes only, and only nodes of the alter's mode have non-NA values for that attribute.

[krivit]

Currently, the (untested) workaround is to impute some single value (e.g., an empty string "") to the focus mode's nodes.