fix: #498 allow Zero-sized strings in VM.

[dorianignee]

The vm call String.new now only checks if size is negative, so that empty strings work as expected.

[DavidSouther]

Who are we to say that a Jack programmer is allowed to create a string that is always empty? (The argument to String.new is the max size, not the initial size. The initial size is always zero. So calling String.new(0) in Jack (or push 0\ncall String.new 1 in VM) gives back a string that can never have any characters in it.)