🏁 trying to fix C2440 error

Error in AppVeyor:

unit-conversions.cpp(441): error C2440: 'initializing': cannot convert from 'nlohmann::json' to 'std::nullptr_t'

(https://ci.appveyor.com/project/nlohmann/json/build/2838/job/wo7im01sq6tvhe9m)

test/src/unit-conversions.cpp

@@ -438,7 +438,7 @@ TEST_CASE("value conversion")
         std::nullptr_t n;
         json j(n);
 
-        std::nullptr_t n2 = j;
+        std::nullptr_t n2 = (std::nullptr_t)j;
         CHECK(n2 == n);
     }