Fix UnnecessaryParentReference with concatenated selector

The `UnnecessaryParentReference` linter would incorrectly report an
error for an `&` concatenated with a suffix, e.g.

  .icon {
    &-small {
      ...
    }
  }

Fix by checking that the `suffix` of the selector is empty, ensuring it
is just a `&`.

Change-Id: Ic40e957480489b20ad5beabe01839359baa40959
Reviewed-on: http://gerrit.causes.com/40992
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Henric Trotzig <henric.trotzig@brigade.com>
Include ampersand in UnnecessaryParentReference description

This will make it more obvious what the linter is complaining about.

Change-Id: I1f73c86d9d970bdc73854dbf150d0f4c1fcf23b0
Reviewed-on: http://gerrit.causes.com/40991
Reviewed-by: Shane da Silva <shane.dasilva@brigade.com>
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Henric Trotzig <henric.trotzig@brigade.com>