text: Title: Axes and content are emptied when writing and re-reading a pd.Panel with np.nan values only
  Body: I'm quite sure, this is not the desired behaviour of the arctic library: When I try to write and re-read a pandas Panel containing only np.nan values, the read object is completely empty i.e. there are no axes or values at all. 
  
  `p = pd.Panel(items=['item'], major_axis=['maj'], minor_axis=['min'])`
  `p`
  
  > <class 'pandas.core.panel.Panel'>
  > Dimensions: 1 (items) x 1 (major_axis) x 1 (minor_axis)
  > Items axis: item to item
  > Major_axis axis: maj to maj
  > Minor_axis axis: min to min
  
  `lib.write('symbol', p)`
  `lib.read('symbol').data`
  
  > <class 'pandas.core.panel.Panel'>
  > Dimensions: 0 (items) x 0 (major_axis) x 0 (minor_axis)
  > Items axis: None
  > Major_axis axis: None
  > Minor_axis axis: None
  
  I'm using arctic '1.27.0' and pandas '0.18.1' within a ubuntu '16.04' docker container.
  