- we have logic to init ._v_zfile and ._v_blk there

- the same for ._v_blkdata - logic to init it is there + it is better to
  set variables right from instance creation, not hoping "it will be set
  outside from master"

  NOTE ._v_blkdata = None means the block was not yet loaded and
  generally fits into logic how ZBlk operates and thus the change is ok.