fix slot fallback update parent

[tanhauhau]

Fixes #4597

[Conduitry]

Do you have a changelog-y description of what this fixes? I'm a bit confused about whether what was described in #4597 was just one aspect of a larger problem, or what exactly this changes or fixes, because both the issue and the test involve <svelte:component> but as far as I can tell, the code change does not.

[tanhauhau]

When <svelte:component> tries to get the update_mount_node via get_update_mount_node, the parent is not null, which is wrong. in the repl, the parent is referring to the <div> element which is defined in the main create_fragment function instead of the fallback create_fragment function

[Conduitry]

This conflicts with #4705 which I merged a little while ago. 4705 changes parent to this and this changes parent to null. I admit I haven't spent too much time thinking about either of these PRs - @tanhauhau and @pushkine do you have any thoughts about why this should be this vs null?

[Conduitry]

#4703 (the issue said to be fixed by #4705) works on this branch, and #4597 (the issue said to be fixed by this PR) is working on master (which has #4705 already merged in).

[pushkine]

it's the parent of the node, so it should be this
the bug depends on the this.parent && this.parent.is_dom_node() check, both fixes will pass
I'm not familiar enough with slots to say whether changing it to null is safe

[tanhauhau]

this instead of null is a better fix.