My guess is right, when using DVD_ID as directory-naming-format when data source is DMM the app is generating empty name
I tried using this for testing purpose
./jav-it catalog -d $MY_DIR --force-update --video-naming-method=original --directory-naming-format='{{CONTENT_ID}}' --data-source=dmm
Now the directories and catalog files are no longer using empty name.
So the remaining bug is just this,
<uniqueid type="DMM"></uniqueid> inside .nfo file failed to generate the video ID from DMM.