Fix parsing external function conditionals when they have zero arguments

[Zacqary]

Fixes a bladeink-compiler bug where

EXTERNAL has_key()
* {has_key()} Show this choice if has_key returns true

would compile correctly, but this wouldn't:

EXTERNAL has_key()
{has_key(): Unlocked.|Locked.}
-> DONE

In this specific case of inline conditionals, the compiler is always looking for internal functions and doesn't consider external. Interpolation works fine.

Functions with args don't seem to have been affected.

Found this bug using a pattern like this; bladeink would silently fail to insert extra_choices:

EXTERNAL show_extra()
-> main
=== main ===
<- base_choices
{ show_extra(): <- extra_choices }
+ [Leave]
    -> DONE
-> DONE
== base_choices ==
+ [Stay]
    -> DONE
== extra_choices ==
+ [Open the safe]
    -> DONE