If I do:

avahi_server_add_service(server, group, 0, AF_UNSPEC,
  "_ssh._tcp", "SSH Server", NULL, NULL, 22, NULL);

Then this happens:

** ERROR **: file rr.c: line 447 (lexicographical_memcmp): assertion failed: (a)
aborting...

Program received signal SIGABRT, Aborted.
0xffffe410 in __kernel_vsyscall ()
(gdb) bt
#0  0xffffe410 in __kernel_vsyscall ()
#1  0xb7a47175 in raise () from /lib/tls/i686/cmov/libc.so.6
#2  0xb7a487d8 in abort () from /lib/tls/i686/cmov/libc.so.6
#3  0xb7b76ea0 in IA__g_logv (log_domain=0x0, log_level=G_LOG_LEVEL_ERROR,
    format=0xb7b9b020 "file %s: line %d (%s): assertion failed: (%s)",
    args1=0xbffff12c "\uffff\026\006\b\uffff\001") at gmessages.c:492
#4  0xb7b76ec6 in IA__g_log (log_domain=0x0, log_level=0, format=0x0) at
gmessages.c:512
#5  0xb7b76f9f in IA__g_assert_warning (log_domain=0x0, file=0x0, line=0,
    pretty_function=0x8061853 "lexicographical_memcmp", expression=0x805fa82 "a")
    at gmessages.c:547
#6  0x080550a5 in lexicographical_memcmp (a=0x6, al=0, b=0x0, bl=0) at rr.c:458
#7  0x08055298 in avahi_record_lexicographical_compare (a=0x8087f68,
b=0x8085ec8) at rr.c:519
#8  0x0804d68d in incoming_probe (s=0x8065008, record=0x8085ec8, i=0x8066b78) at
server.c:231
#9  0x0804e474 in handle_query (s=0x8065008, p=0x8088608, i=0x8066b78, a=0xbffff270,
    port=5353, legacy_unicast=0) at server.c:572
#10 0x0804ea20 in dispatch_packet (s=0x8065008, p=0x8088608, sa=0x0, iface=6,
ttl=255)
    at server.c:683
#11 0x0804ebc9 in work (s=0x8065008) at server.c:722
#12 0x0804ed1f in dispatch_func (source=0x0, callback=0, user_data=0x0) at
server.c:760
#13 0xb7b6ed0f in g_main_dispatch (context=0x80650c0) at gmain.c:1947
#14 0xb7b6fcb5 in IA__g_main_context_dispatch (context=0x80650c0) at gmain.c:2497
#15 0xb7b6ffd7 in g_main_context_iterate (context=0x80650c0, block=1, dispatch=1,
    self=0x80674a0) at gmain.c:2578
#16 0xb7b7051e in IA__g_main_loop_run (loop=0x8085878) at gmain.c:2782
#17 0x08049fa8 in main (argc=1, argv=0xbffff564) at avahi-test.c:212

As I understand it, NULL TXT records are allowed.