fix: make embed color optional

[erics118]

made embed::color a std::optional
Discord does not use 0x000000 as the default embed color, so this actually does result in a visual change to the embeds created that dont have a color, (also has a more noticable change in light mode)

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[Mishura4]

Might as well change

DPP/src/dpp/message.cpp

Line 621 in b6854d3

color = int32_not_null(j, "color");

as well so it doesn't fill it to 0 even if the field is present