comment update
if A or AAAA is nil return empty string

Now it returns <nil> for nil value, which can not be parsed
back to the record.
Allow empty rdata in records

Empty or no rdata is allowed for dynamic updates, so test if this
works for packing/unpacking. It only fails for TSIG (which is
never seen in zone files), SOA (which is not seen like this in dyn.
updates) and WKS (just an old record).
better comment