chelseadzdr

MSVC: make #4526 pass (use #if for optional) - fixes #3859; re-amalgamate

7

### Changes made - pulled pr #4526 and amalgamated - merged into develop and fixed conflicts, kept most recent version of from_json.hpp (removed extra JSON_USE_IMPLICIT_CONVERSIONS that broke C++11 builds). -...

Fix NLOHMANN_DEFINE_TYPE_* macros for zero-member types

3

This pull request solves the issue #4041 using the changes made in the closed PR #4323 - [x] The changes are described in detail, both the what and why. -...