Source empty if script contains only comments

[czosel]

I found another edge case: the source attribute of the program node is empty, if the file contains nothing but comments:

<?php
//test

{
  "kind": "program",
  "loc": {
    "source": "",

vs.

<?php
//test
echo 1;

{
  "kind": "program",
  "loc": {
    "source": "<?php\n//test\necho 1;",

[ichiriac]

need to test, but should be fixed in 3.0.0-prerelease.3