Enhance `await` usage warnings with null coalescing operator

**Brief description:**

When using the null coalescing operator on an async delegate there is no warning that `await` is missing. Example:


First, we can see that `initialize()` triggers a warning, but not `initialize?.Invoke()`.
Besides, another warning could be displayed if someone simply adds `await` while using the operator as the result could be `null`.

**Languages applicable:**

I am not sure.

**Code example that the analyzer should report:**

1. `initialize?.Invoke()` -> missing await
2. `await initialize?.Invoke()` -> don't use await with coalescing operator