macro select no branch else only

[GilShoshan94]

Motivation

Closes #6338

Solution

It's the propesed "Alternative 3" "Alternative 5" described in here.
It simply adds a special case for when the select! has no branches except for the else fallback branch (for the case with and without biased mode). It will simply expand to the code in $else.

I added 2 tests case too to cover it.

[Darksonn]

Thank you.