autobahn requires zero length tx allowed

autobahn requires zero length tx allowed

Again we treat user code sending zero length things as a bug in user code.

But Autobahn insists to be able to do it, so now we allow it.

That buys us a pass on Autobahn test 1.1.1 (the first of a bazillion)

Reproduce with

libwebsockets-test-echo --client localhost --port 9001 -u "/runCase?case=1&agent=libwebsockets" -v -d 65535 -n 1

Signed-off-by: Andy Green <andy.green@linaro.org>