Fix build of math/vtk6 with Clang6 on -CURRENT.

The definition of NULL has changed in C++, and conversions have
gotten stricter, so using NULL where you mean 0 (or false) is
no longer an option. Detected by fallout from updating CMake 3.12.

No PORTREVISION bump because everywhere where it compiled previously
sees no change, and where it didn't compile before it does now
with the same result.

Reported by: tcberner