Null references used for type dispatch

[dawagner]

Here: https://github.com/thrust/thrust/blob/417d78471dadefa3087ff274e64f43ce74acfd3d/thrust/detail/reference.inl#L120 and at some other places in this file and also in its associated .h file, null pointers are created and passed to functions which are known to dereference them.

According to the comments and by looking at how they are ultimately used, it seems that we only care about the type. Is there anything that prevents using meta-programmation instead of passing actual, invalid values?