Fix nil parameters and returns

[XmiliaH]

The VM discarded arguments and returns with nil values in some cases such as the following script

local function multiret()
    return 1, nil, 2, nil, nil, 5, nil, nil
end

local function r(...)
    return ...
end

local function t(...)
    return r(...)
end

print(r(t(multiret())))

This was mostly caused by the usage of table.insert which discards nil values and the pattern unpack({...}) which uses # for the length of the table which also discards nil values.