BUG: Fixed bug in len(DataFrame.groupby) causing IndexError when there's a NaN-only column (issue11016)
Fixed #11016 

@jreback Could you help me with this conflict? 
 don't put the whatsnew note at the very end of bug fixes. stick it where their is blank space. at the end always causes conflicts, much much more rare when its not.
 Thanks. I had trouble rebasing it. It has conflict that I failed to fix. I've created a new PR. Sorry for inconvenience. 
