rule-syntax-check: fix handling of runaway strings in comma splitting (#8298)

rule-syntax-check: fix handling of runaway strings in comma splitting (#8298)

A runaway string should still be returned by the code that splits on
commas, so add a '?' to the regex so that the last '"?' in a string
still produces a valid block for the split code.

Tested:

  ACTION=="remove\"GOTO=""

Which then produced:

  $ test/rule-syntax-check.py src/login/70-uaccess.rules
  # looking at src/login/70-uaccess.rules
  Invalid line src/login/70-uaccess.rules:10: ACTION=="remove\"GOTO=""
    clause: ACTION=="remove\"GOTO=""