outroing is not defined with nestedTransitions: true

[Rich-Harris]

With the nestedTransitions: true option, it's possible to end up in a situation where one branch of an if block potentially has outros, but another doesn't, and Svelte checks for a non-existent outroing variable in the block that doesn't upon update.

{#if y}
	<Foo/>
{:else}
	<span>{x}</span>
{/if}

Noticed a couple of other things:

• we don't really need to update the if block at all; perhaps that should be short-circuited anyway
• transitionManager gets added even though there's no transitions, because it's trying to group the outros (in case <Foo> has some). I wonder if we can separate groupOutros from transitionManager so that it doesn't get pulled in