(supersedes #6652)

Fixes #6543

This omits trailing undefineds when applying external functions with optional arguments, when the optional argument itself is not provided (or is None). Only applies to external calls, because that's the only place where it could matter.

Some more background: ReScript v11 with uncurried mode allows us to write much more ergonomic external function bindings because we can have optional arguments where we don't need a trailing unit to apply the function itself. But, this comes with a potential problem. All arguments, whether they're actually supplied or not, would be printed as undefined in the resulting JS. External JS calls might care about the number of args sent into it, so someJsFn(1) and someJsFn(1, undefined) is not always treated the same. Several JS APIs can break if provided the wrong amount of arguments unless intended.

cc @cristianoc what do you think about this?