Malformed generated code in a situation I can't describe

[Conduitry]

Describe the bug
For

{#if $foo}
{:else}
	{#if bar()}
		<Baz/>
	{/if}
{/if}

or alternatively

{#if $foo}
{:else if bar()}
	<Baz/>
{/if}

Svelte generates code like if ((show_if == null) || ) show_if = !!(bar()).

To Reproduce
Compile the above, and look at the invalid code that's generated.

Expected behavior
Presumably the || part just shouldn't be there, but I haven't checked whether that code would do what is expected.

Information about your Svelte project:
Svelte 3.12.1

Severity
Medium-low-ish, probably. This seems to occur in a fairly specific situation.

Additional context
Quite possibly this will be fixed by #3539, but I wanted a record of this repro anyway.

[Conduitry]

An equivalent bug seems to be currently present in that branch. I'm getting an error about reducing an empty array here, but at least a compile-time exception is better than generating invalid code.

[ghost]

@Conduitry, possibly also related to #3588?

[joshleblanc]

@dkondrad this issue also appeared in 3.9.1, it compiles fine in prior versions.