It is possible to save a Journey Log without some data inserted into it.
In the section Admin > Operator settings > Flight editing you need to mark a checkbox 'Allow not completed JL to be entered' and save.
You can enter completed flight times to Journey Log (BLOFF, T/O, LDG, BLON) and leave fuel data unfilled, then save changes.
Or, you can enter not completed fligt times (only T/O and LDG or only BLOFF and BLON) leaving fuel data unfilled and also save the Journey Log.
Not completed Journey log is marked in yellow in the 'Flights List' as . Once it's filled in completely it becomes green