[fix]: missing null value of append_styles in init function

[zhiyue-yi]

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Hi. When I was using svelte-navigator, I encountered this issue: Uncaught TypeError: append_styles is not a function and there was an issue created in svelte-navigator here: mefechoel/svelte-navigator#47

Here is the repo to reproduce the issue: https://github.com/ZhiyueYi/web-toolbox

I think the issue was introduced by this MR: #5870 which didn't handle default case of append_styles well at src/compiler/compile/render_dom/index.ts#L546 in this MR