[OpenMP][test] Properly check for NULL (#139364)
Only 2 `openmp` testsuite failures remain on Solaris/amd64. They are due
the same issue: the tests in question assume `NULL` pointers to be
printed as `(nil)` while the rest of the testsuite uses `[[NULL]]` for
that.
This patch changes them to follow suit.
Tested on `amd64-pc-solaris2.11` and `x86_64-pc-linux-gnu`.
