Min/max values and distinct counts are already optional, so let's make
the null counts optional as well. This will be helpful for NG to deal w/
partial statistics (e.g. we only populate stats for the time column).
Note that the total count is still mandatory, but we normally have the
chunk/file-level row count at hand.
Andrew Lamb
Carol (Nichols || Goulding)
Marco Neumann
Raphael Taylor-Davies