lei_xsearch: use curl -d '' for nginx compatibility

lei_xsearch: use curl -d '' for nginx compatibility

It appears Content-Length and/or Content-Type headers are
required by nginx with POST requests.

varnish alone doesn't have this requirement and my (perhaps
lossy) reading of RFC 2616, 7230, 7231 didn't note this, either.

In any case, we must support nginx even if it's overly strict.

Reported-By: Kyle Meyer <kyle@kyleam.com>
Link: https://public-inbox.org/meta/87v9bmswkh.fsf@kyleam.com/