resolved: do not try to copy empty NSEC types bitmaps

dns_resource_record_copy() assumes that NSEC types bitmap is non-empty
which results in a null pointer dereference inside bitmap_copy() in some
cases. Fix this by calling bitmap_copy() conditionally.

(cherry picked from commit 1f00a50c)
(cherry picked from commit fc7be6db)
(cherry picked from commit 3422b16e)