To intercept https pages, you need to create a Man-in-the-Middle and have your own SSL certificate. That is not something we support.
NULL is the default setting, for thr reasons discussed in the documentation. In the absence of any setting in /etc/pihole/pihole-FTL.conf, NULL will be used. Since you didn't have an alternate blocking mode set, the output line in the debug log shows that NULL was set when pihole-FTL started. If you made a subsequent change in the configuration, either it was not saved of pihole-FTL was not restarted after the changes.