[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`.

GitOrigin-RevId: 6fd3381639aa6ae86f119752f1326641232014de