There is no reason for pixelserv-tls to respond to an empty http or https request for the reason there's nothing to act upon for the built in web-server.
pixelserv-tls reacts to certain requests like ad pictures, js files and such.
It will respond with the stats request on both protocols:
http://<pixelserv IP>/servstats
http
s://<pixelserv IP>/servstats
However, to respond on the secure http
s protocol, you need to import the certificate into your device/browser.
The procedure is described in the wiki, look under import:
https://github.com/kvic-z/pixelserv-tls/wiki/Create-and-Import-the-CA-Certificate
To get the cert off of the router, you can use the AB backup function, it includes the cert needed or get the ca.crt from /opt/var/cache/pixelserv/
Edit: I keep forgetting to add that you can test it with any blocked domain in AB-Solution, as for example with doubleclick.net:
https://doubleclick.net/ad.jpg
It will throw an error if this is the first time the domain has been requested, pixelserv-tls needs to create the cert first for that domain. Subsequent requests for
https://doubleclick.net/ad.jpg will show you the 1x1 pixel.