resolved: remove inappropriate assert()

resolved: remove inappropriate assert()

A NULL Bitmap object is by all our code considered identical to an empty
bitmap. Hence let's remove the entirely unnecessary assert().

The assert() can be triggered if debug monitoring is used an an empty
NSEC or NSEC3 RR is included in an answer resolved returns.

it's not really a security issue since enabling debug monitoring is a
manual step requiring root privileges, that is off by default. Moreover,
it's a "clean" assert(), i.e. the worst that happens is tha a coredump
is generated and resolved restarted.

Fixes: #25449