Fix empty return statements not being emitted in bytecode.

[BenJilks]

Fixes issue #34.

To determine if a block has a return statement, it was checking if retExps is empty. However, if an empty return statement was given (for example, in the code below), this would also be empty. Meaning the RETURN bytecode was never emitted.

function test()
    if true then
        return
    end
    print("should not get here")
end

This PR makes block retExps optional. This value being null means there was no return statement at the end of the block. It being an empty list, means an empty return statement.