I think that this folder holds the scan-by-location information, such as the DVB-T muxes for a particular transmitter. I can't say why it's empty without doing some homework.
You're using master, yes? I'm still on 3.4.something, and there has been some significant rewrite between then and now; I also don't have DVB-C, so I don't think I can help with the tuning. Sorry!
My guess is that your empty window is related to the data/dvb-scan bit. What I don't understand is that this fork:
https://github.com/PiratJones/tvheadend/tree/master/data
has something, but the master code:
https://github.com/tvheadend/tvheadend/tree/master/data
... doesn't.
I haven't recompiled for some time, but my git checkout from last March certainly has the dvb-scan files - but I also can't find them on the release/3.2 and release/3.4 branches. Maybe something's changed in master and how the code handles this; maybe something's broken. If you don't mind breaking things, perhaps copy them from the fork above and see if that fixes things?
EDIT
I also wonder whether installing dvbutils or dvbscan or something would provide the data files, and you could look from there. There seem to be many DVB-C files in that fork, so maybe you can copy the one you need.