Emptytheme: Stop clearing presets

[kjellr]

Emptytheme contains the following code in its theme.json, which is a helpful starting place for theme authors to populate:

		"color": {
			"gradients": [],
			"palette": []
		},
		"typography": {
			"fontFamilies": [],
			"fontSizes": []
		},

... but the downside is that this clears out those presets, and makes their associated tools unavailable (as noticed by @jasmussen here).

I think it makes sense to remove them.

[MaggieCabrera]

I agree, I thought about it but never got to make a PR, LGTM!

[jasmussen]

Nice one. I was about to ask for the default color palette to also work, but you're on top of it. Sweet!