tweak(mono-rt2): Return Complete Coroutine when no data is returned

[niekschoemaker]

This allows awaiting exports without a null check.

Goal of this PR

In the current implementation of exports exports might return null instead of a coroutine, which is the case for exports which execute within a tick (so no wait calls) and don't return a value.
Simply returning a Completed Coroutine with a null value fixes this issue.

How is this PR achieving the goal

By returning a completed coroutine with a null value.

This PR applies to the following area(s)

ScRt: C# v2

Successfully tested on

Checklist

• Code compiles and has been tested successfully.
• Code explains itself well and/or is documented.
• My commit message explains what the changes do and what they are for.
• No extra compilation warnings are added by these changes.

[niekschoemaker]

Another one which is somewhat opiniated.

In my opinion being able to call await on the export without a null check and then handling null values after the await seems like a cleaner solution than:

var result = Exports["test", "test"]();
if (result != null) {
    await result;
}