Factoring out the workaround to access last variadic parameter.

Indeed when some parameter are BOOST_PP_EMPTY the last parameter is
accessed correctly but results in a compilation error which doesn't stop
some compiler (e.g. g++) to go through the preprocessing pass and still
compile the binary correctly. But to avoid the error message I used a
workaround which behaves better. I preferred factoring it out to make
clear that there are some reason why this element is strangely accessed.