Add null UUID definition to use for comparing

[martincapello]

This PR just defines a "null" UUID constant by introducing a static const member to the Uuid class.

[aseprite-bot]

clang-tidy review says "All clean, LGTM! 👍"

[martincapello]

This is not needed anymore actually...so feel free to do whatever you want with this.

[dacap]

I'll close this as it's not required and the expected behavior is that just creating a new Uuid() with the default contructor will create an empty Uuid():

laf/base/uuid_tests.cpp

Lines 13 to 19 in 412139c

	TEST(Uuid, Empty)
	{
	Uuid uuid;
	for (int i = 0; i < 16; ++i) {
	EXPECT_EQ(0, uuid[i]);
	}
	}

Generally uuid == Uuid() should be enough to check if an UUID is empty, but we could add a new member function bool empty() const if needed.