Fix Indentation linter handling of comments

The `Indentation` linter would error on SCSS where a comment preceded
another line of code, e.g.

    // A comment
    $var: 1;

This was caused by the code not checking if the `source_range` was set
for that node, which is the case for comment nodes (they only return a
value for `line`).

Change-Id: I7afd16b2b6823902db6c81b9ea5da1c07527b4b3
Reviewed-on: http://gerrit.causes.com/39181
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Shane da Silva <shane.dasilva@brigade.com>