Fix: asserts unintentially being partially disabled in release-builds (#11268)

The nlohmann-json header file includes assert.h, which rudely resets
the assert macro to what that header thinks is right. As we set the
assert macro to be active with release builds when WITH_ASSERT is
active, this means that every file including nlohmann-json has their
asserts disabled (for release-builds) but files that don't do no.

Let's avoid this issue, by telling nlohmann to not include assert.h.