Properly reject zero length ALPN in commatxt_fromtext

Properly reject zero length ALPN in commatxt_fromtext

ALPN are defined as 1*255OCTET in RFC 9460.  commatxt_fromtext was not
rejecting invalid inputs produces by missing a level of escaping
which where later caught be dns_rdata_fromwire on reception.

These inputs should have been rejected

	svcb in svcb 1 1.svcb alpn=\,abc
	svcb1 in svcb 1 1.svcb alpn=a\,\,abc

and generated 00 03 61 62 63 and 01 61 00 02 61 62 63 respectively.

The correct inputs to include commas in the alpn requires double
escaping.

	svcb in svcb 1 1.svcb alpn=\\,abc
	svcb1 in svcb 1 1.svcb alpn=a\\,\\,abc

and generate 04 2C 61 62 63 and 06 61 2C 2C 61 62 63 respectively.