[array] Add constructor taking std::nullptr_t Not all impls implement nullptr_t in a way that would automatically convert to what we were accepting. In file included from ../src/test-map.cc:27: ../src/hb-map.hh:44:22: error: no viable conversion from 'nullptr_t' to 'const hb_array_t<const char>' static constexpr K INVALID_KEY = kINVALID; ^ ~~~~~~~~