Currently if a null appears in a table column, for instance in data
returned by postgres, sorting on that gives an arbitrary order. This
is due to null being neither greater or less than any string, which
makes the sort unstable.

Change the table sort function to compare on nullness first. Note
this is a slight behaviour change for numbers, which would otherwise
treat null and 0 as equivalent.

Signed-off-by: Martin Packman <gzlist@googlemail.com>