Remove duplicate check for NullPINs

Use hasNKSNullPin() and hasSigGNullPin() instead of checking the PIN
states. In particular, this makes the override for testing via
environment value work.

(cherry picked from commit 2f6852e436818f63d802b36793d81c7d3d6f358c)

• GnuPG-bug-id: T7018