Prelisp cannot handle empty lists

[GlowingScrewdriver]

Prelisp recursively tests the first element of each list to determine whether it is an unquote or unquote-splicing expression. This test causes an IndexError when an empty list is being processed. Consider the following macro call:

,@(array-init arr () 5)

Prelisp will run into an error while analyzing the second argument to the macro:

  File "/home/gsd-vnai/Desktop/llama.lisp/src/backend/tests/prelisp/../../prelisp.py", line 17, in preprocess
    if expr[0] == "unquote":
       ~~~~^^^
IndexError: list index out of range

[GlowingScrewdriver]

May be fixed by #91

[chsasank]

Create a PR with regression test