Fixes#2747
I admit I don't fully understand this. But somehow, doing `var x: ?T =
undefined` in release fast mode makes `x` act as if its unset. I am
guessing since undefined does nothing to the memory, the memory layout
is such that it looks null for zeroed stack memory. This is a guess.
To fix this, I now initialize the type `T` and set it onto the optional
later. This commit also fixes an issue where calling `parseCLI` multiple
times on an optional would not modify the previous value if set.
4ef2240618