Records with empty rdata causes pack() to fail

[peteralm80]

A DNSRecord instance such as

<DNS Header: id=0x9d1a type=QUERY opcode=UPDATE flags= rcode='NOERROR' zo=1 pr=1 up=3 ad=0>
<DNS Question: 'xxx.net.' qtype=SOA qclass=IN>
<DNS RR: 'yyy.xxx.net.' rtype=CNAME rclass=None ttl=0 rdata=''>
<DNS RR: 'yyy.xxx.net.' rtype=AAAA rclass=* ttl=0 rdata=''>
<DNS RR: 'yyy.xxx.net.' rtype=A rclass=* ttl=0 rdata=''>
<DNS RR: 'yyy.xxx.net.' rtype=A rclass=IN ttl=1200 rdata='192.168.1.2'>

will fail to pack() with the following error:

>>> r.pack()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/peter/.local/lib/python3.12/site-packages/dnslib/dns.py", line 335, in pack
    rr.pack(buffer)
  File "/home/peter/.local/lib/python3.12/site-packages/dnslib/dns.py", line 888, in pack
    self.rdata.pack(buffer)
    ^^^^^^^^^^^^^^^

AttributeError: 'str' object has no attribute 'pack'

I believe the source of the error is https://github.com/paulc/dnslib/blob/724f94e52cbc030211d0f86b2271aa2f4874d704/dnslib/dns.py#L831