Can't run a trial with only validation steps

[MattPainter01]

Running:

Trial(...).for_val_steps(10).run()

results in the TypeError:

...
  File "/home/matt/PycharmProjects/PyTorch-bink/torchbearer/trial.py", line 841, in _fit_pass
    for state[torchbearer.BATCH] in (range(state[torchbearer.STEPS]) if state[torchbearer.STEPS] != -1 else itertools.count()):
TypeError: range() integer end argument expected, got NoneType.