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.

[jasmussen]

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