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: I600dcf11b686d8d72b2bcdac88b7158288c68105
Reviewed-on: https://chromium-review.googlesource.com/265395Reviewed-by: Olli Etuaho <oetuaho@nvidia.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Tested-by: Olli Etuaho <oetuaho@nvidia.com>