common: Fix logic for certain recsel conditions.

* common/recsel.c (recsel_select): Change processing of NULL values.
* common/t-recsel.c (run_test_2): Adjust for this change.  Also a type
fix for s/"letter"/"letters"/.

The getval function may return NULL which indicates that there is no
useful value available. For example because the propertyname is not
defined for some external context (e.g. in gpg the packet type). This
also required to fix the test for boolean tests of a non existing
property name.

• Reported-by: shniubobo at gnupg-users on 2025-04-18.