resultspec: treats None as smaller to anything

This avoids useless 500 when the data set contains
null datetimes
test for null datetime sorting
more tests for resultsspec's nonecmp

Signed-off-by: Pierre Tardy <pierre.tardy@intel.com>
Merge pull request #1313 from tardyp/t2978

Fix: Loading the waterfall while a build is in progress return 500

Fixes ticket:2978