In case windowed expressions - plain columns which contain null values show default values - even when NullHandling#sqlCompatible() is true

   SELECT
     l2,
     MIN(l2) OVER()
   FROM druid.numfoo
   WHERE l2 is null or l2 = -1111

returns

0,null

instead of the expected:

null,null

The issue was with ColumnAccessor#getObject implementations; for which the apidoc stated that when isNull returns true it should return null