CMake: demote -Wnullability-completeness to warning for unity buildsHEADdev

Unity builds currently fail on macos with -Werror, due to
-Wnullability-completeness. It seems that apple's system headers don't
have every NS_ASSUME_NONNULL_BEGIN followed by a NS_ASSUME_NONNULL_END,
which seems to leak into our code.
Demoting the errors to a warning to make -Werror builds pass again.

Pick-to: 6.10
Change-Id: I70d8ae33bf57b87feac7bb33b929b74f8cd60504
Reviewed-by: Tor Arne Vestbø <[email protected]>
Reviewed-by: Alexey Edelev <[email protected]>

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