Add std::nullptr_t constructor overload for ref_ptr

[AnyOldName3]

Adds a std::nullptr_t constructor overload for vsg::ref_ptr, allowing ref_ptr to be constructed from nullptr. This is more consistent with other smart pointer implementations, like std::shared_ptr, and makes some things less verbose that previously needed an explicitly-default-constructed-explicitly-typed ref_ptr for templates to resolve more concise.

I've demonstrated that change in a few places that I'd already worked on, but there are more that could benefit if it's more consistent with the project's desired code style. A repo-wide regex search for ref_ptr<[^()]*>\(\) will reveal them. Even if it's not wanted for the VSG itself, the overload is still a good thing to have.

[robertosfield]

I merged not expecting any problems but now get LOTS of warnings and errors when building on Kubuntu 22.04 with gcc 11.4.0. I have had to revert the merge, once fixes can be found I will be happy to merge.

The warnings/errors are in the form:

home/robert/Dev/VulkanSceneGraph/include/vsg/core/ref_ptr.h:63:16: warning: unnecessary parentheses in declaration of ‘nullptr_t’ [-Wparentheses]
63 | ref_ptr(std::nullptr_t) noexcept :
| ^~~~~~~~~~~~~~~~
/home/robert/Dev/VulkanSceneGraph/include/vsg/core/ref_ptr.h:63:16: note: remove parentheses
63 | ref_ptr(std::nullptr_t) noexcept :
| ^~~~~~~~~~~~~~~~
| - -
/home/robert/Dev/VulkanSceneGraph/include/vsg/core/ref_ptr.h:63:17: error: invalid use of ‘::’
63 | ref_ptr(std::nullptr_t) noexcept :
| ^~~
/home/robert/Dev/VulkanSceneGraph/include/vsg/core/ref_ptr.h:63:31: error: expected ‘;’ at end of member declaration
63 | ref_ptr(std::nullptr_t) noexcept :
| ^
| ;
/home/robert/Dev/VulkanSceneGraph/include/vsg/core/ref_ptr.h:63:33: error: expected unqualified-id before ‘noexcept’
63 | ref_ptr(std::nullptr_t) noexcept :
| ^~~~~~~~

[AnyOldName3]

I forgot #include <cstddef> (because apparently MSVC doesn't need it, despite it being required by the standard). If you don't want ref_ptr.h to include headers, decltype(nullptr) is guaranteed by the standard to be the same type.

[robertosfield]

decltype(nullptr) is a bit hacky but better than including another header.