WithDecoration: fixed crash when Palette is null

[michaeldgg2]

In current code, when Palette is null, WorldRenderer.Palette is invoked with empty string and in conjunction with how palettes are retrieved in HardwarePalette:

public IPalette GetPalette(string name)
{
	if (mutablePalettes.TryGetValue(name, out var mutable))
		return mutable.AsReadOnly();
	if (palettes.TryGetValue(name, out var immutable))
		return immutable;
	throw new InvalidOperationException($"Palette `{name}` does not exist");
}

results in crash. This PR changes how the palette name is constructed so that with Palette == null it doesn't result in empty string.

[PunkPun]

wouldn't it be better for WorldRenderer.Palette to check string.IsNullOrEmpty instead?

[michaeldgg2]

It would be a better solution, but my intention was to make just the fix for WithDecoration (as it's currently blocking us). Adding check to WorldRenderer.Palette means that the null check, that is currently used throughout the code base, would be redundant and thus should be removed. That means bigger PR, which means longer review time.

But if you want, I can close this and make that bigger PR.

[PunkPun]

It wouldn't really be a bigger PR. And I mean replacing the null check with a is null or empty check

[Mailaender]

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