Support partition pruning on boolcol IS [NOT] UNKNOWN
While working on
4c2369ac5, I noticed we went out of our way not to
support clauses on boolean partitioned tables in the form of "IS
UNKNOWN" and "IS NOT UNKNOWN". It's almost as much code to disallow
this as it is to allow it, so let's allow it.
Discussion: https://postgr.es/m/CAApHDvobKtcN6+xOuOfcutfp6T7jP=JPA9y3=MAEqnuKdDsQrw@mail.gmail.com