Revert "Revert "Use nullptr consistently in TIntermConstantUnion::fold""
Revert "Revert "Use nullptr consistently in TIntermConstantUnion::fold""

Revert of revert as the new changes that use nullptr in fold function
are already in.

This reverts commit b775778a050739c4d802da8db7edb8c865602950.

Change-Id: Ifbdb35264132c66e5d1f2379932839b99291c54a
Reviewed-on: https://chromium-review.googlesource.com/266803
Reviewed-by: Olli Etuaho <38ae6c0e2772a20e8e989f80cb8c99d274e44242@nvidia.com>
Reviewed-by: Jamie Madill <7e492b4f1c8458024932de3ba475cbf015424c30@chromium.org>
Tested-by: Olli Etuaho <38ae6c0e2772a20e8e989f80cb8c99d274e44242@nvidia.com>

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