Fix Indentation handling of comments in @at-root

The linter would crash for code like the following:

    @at-root {
      // A comment
      ...
    }

This was due to the fact that comments don't have source ranges. Fix by
explicitly checking if `source_range` is not nil before attempting to
access it.

Change-Id: I8b1518458381689178b402fc8b9eb3c8dfa13100
Reviewed-on: http://gerrit.causes.com/44888
Tested-by: jenkins <jenkins@brigade.com>
Reviewed-by: Shane da Silva <shane.dasilva@brigade.com>