Newly added code in TIntermConstantUnion::fold uses nullptr where as
remaining part of the function uses NULL. This change removes NULL and
consistently uses nullptr in complete function.

Change-Id: Ieaada3cf46bf3e16d3ef91b300c86537f778c33f
Reviewed-on: https://chromium-review.googlesource.com/264632Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Olli Etuaho <oetuaho@nvidia.com>
Tested-by: Olli Etuaho <oetuaho@nvidia.com>