Writer: Properly handle export of features that are empty (all-gated-out) or all-nan.

[paulmueller]

Traceback (most recent call last):
  File "shapeout2\gui\export\e2data.py", line 46, in done
    self.export_data()
  File "shapeout2\gui\widgets\wait_cursor.py", line 31, in wrapper
    ret = func(*args, **kwargs)
  File "shapeout2\gui\export\e2data.py", line 89, in export_data
    ds.export.hdf5(
  File "dclab\rtdc_dataset\export.py", line 318, in hdf5
  File "dclab\rtdc_dataset\export.py", line 470, in store_filtered_feature
  File "dclab\rtdc_dataset\writer.py", line 295, in store_feature
  File "dclab\rtdc_dataset\writer.py", line 662, in write_ndarray
  File "numpy\lib\nanfunctions.py", line 350, in nanmin
  File "numpy\core\fromnumeric.py", line 2970, in amin
  File "numpy\core\fromnumeric.py", line 88, in _wrapreduction
ValueError: zero-size array to reduction operation minimum which has no identity