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 1f00a50c695fe3b55dee38fbd02a902a6c703c87)