Fix check for uninitialized condition in compound if block with static condition, needs_update and outros

[dimfeld]

Fixes #4840

With a static if condition and both needs_update and outro support enabled on the if block, the check for condition (usually rendered as show_if in the generated code) is checked against -1 to see if the static function should be run, but the variable starts out uninitialized and so it doesn't get checked. This PR changes it to check for ${condition} == null instead, the same check used elsewhere in the file.

Here's an annotated example from the original REPL:

<script>
export let x;
function test() { return true; }
</script>

{#if test()} <!-- Static condition -->
    first
{:else if x} <!-- Turns on needs_update for the block -->
    second
{:else}
    <Component2 size=16 /> <!-- Include a component to enable outro handling -->
{/if}

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