MRG, BUG: Fix bug with no picks

[larsoner]

Fixes:

Traceback (most recent call last):
  File "/home/larsoner/python/matplotlib/lib/matplotlib/cbook/__init__.py", line 256, in process
    func(*args, **kwargs)
  File "/home/larsoner/python/mne-python/mne/viz/_figure.py", line 662, in _keypress
    self._redraw()
  File "/home/larsoner/python/mne-python/mne/viz/_figure.py", line 2122, in _redraw
    self._update_data()
  File "/home/larsoner/python/mne-python/mne/viz/_figure.py", line 1925, in _update_data
    this_data = _filtfilt(
  File "/home/larsoner/python/mne-python/mne/filter.py", line 433, in _filtfilt
    x, orig_shape, picks = _prep_for_filtering(x, copy, picks)
  File "/home/larsoner/python/mne-python/mne/filter.py", line 272, in _prep_for_filtering
    picks = _picks_to_idx(x.shape[-2], picks)
  File "/home/larsoner/python/mne-python/mne/io/pick.py", line 1061, in _picks_to_idx
    raise ValueError('No appropriate channels found for the given picks '
ValueError: No appropriate channels found for the given picks (None)

by short-circuiting the filter loop (over filterable time segments) when none of the currently shown channels are data channels.

[drammock]

thx @larsoner