-Wzero-as-null-pointer-constant leads to compiler error in g++

As of boost 1.72, I'm getting "error: zero as null pointer constant" when using e.g. `boost::chrono::milliseconds`. This can IMO be fixed by using `nullptr` instead.

I've spotted the following places, but there might be more:
* duration.hpp:458
* duration.hpp:481
* duration_put.hpp:216
* time_point.hpp:185

Start with duration.hpp at lines 458 and 481, duration_put.hpp at line 216, and time_point.hpp at line 185, then search nearby chrono sources for similar zero pointer constants. Build or test the affected Boost.Chrono examples with g++ and -Wzero-as-null-pointer-constant; done means the reported compiler errors no longer occur.

Written by the indexing model from the issue text.