MINOR: httpclient: don't add body when istlen is empty

MINOR: httpclient: don't add body when istlen is empty

Don't try to create a request with a body in httpclient_req_gen() if the
payload ist has a ptr but no len.

Sometimes people have their httpclient stuck because they use an ist
with a data ptr but no len. Check the len so this mistake doesn't block
the client.