Handle java.lang.Void return type in implicit mode

[jkaving]

Since null is the only allowed return value for a method declared with java.lang.Void return type, it doesn't make much sense to throw an IllegalStateException for these in implicit mode.

Fixes #46

[jkaving]

Note that this will affect explicit mode as well. If you declare a method like this:

@NotNull
Void call() {
    return null;
}

you will no longer get an exception thrown.

But adding @NotNull to a method that returns Void doesn't make much sense anyway (since null is the only allowed return value), so I think that's ok.