ci: enable clang-tidy in macOS job, use nullptr

[Sjors]

The first commit reproduces #261:

/Users/runner/work/libmultiprocess/libmultiprocess/src/mp/util.cpp:84:25: error: use nullptr [modernize-use-nullptr,-warnings-as-errors]
   84 |     pthread_threadid_np(NULL, &tid);
      |                         ^~~~
      |                         nullptr
2 warnings generated.
Suppressed 1 warnings (1 in non-user code).
Use -header-filter=.* or leave it as default to display errors from all non-system headers. Use -system-headers to display errors from system headers as well.
1 warning treated as error

https://github.com/bitcoin-core/libmultiprocess/actions/runs/23348264051/job/67919536711?pr=262

Using clang-tidy in the macOS job increases the runtime from ~1 minute to 2.5 minutes, which shouldn't be a problem.

The second commit fixes it.

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Concept ACK	pinheadmz

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[pinheadmz]

concept ACK, will test locally later today.

TIL DMP_ENABLE_CLANG_TIDY is a cmake option. Does that work in bitcoin core as well?

[hebasto]

TIL DMP_ENABLE_CLANG_TIDY is a cmake option.

To be pedantic, the variable name is MP_ENABLE_CLANG_TIDY, and -D is a CMake option to set a variable.