Fix a warning about an enumeration value not being handled in a switch statement

[nasso]

Accessor::ComponentType::None was not explicitly handled by the switch statement in fx::gltf::detail::WriteAccessorMinMax(nlohmann::json&, Accessor const&) (at include/fx/gltf.h:1318).

This caused a warning on some compilers with the right flags. Here's an example with GCC 10:

/[...]/include/fx/gltf.h:1318:20: warning: enumeration value ‘None’ not handled in switch [-Wswitch]
 1318 |             switch (accessor.componentType)
      |                    ^

This PR addresses this by explicitly ignoring the missing value, keeping the old behaviour while silencing the warning.