i have got the same problem (php 5.6 ) . On NC 11 quota of 0 was OK. Now in NC 12 quota of 0 can not be set via the gui.
If the quota is set on database table for a user with the value 0 it works. Upload of files is not possible anymore and a info box pops up “storage is full. No upload of files possible”
It would be nice to make it possible to set the quota via the GUI to zero. (like in NC 11)