Fix crash in SingleLinePerSelector for interpolation

The `SingleLinePerSelector` linter would crash on selectors containing
only interpolation, e.g.

    #{$variable} {
      ...
    }

This was caused by `inject` returning `nil` on an empty array. We change
the implementation to return an empty string in this case.

Change-Id: Iabe6b7f87a4fa1fd7bd92ab2f8ce783495fce2df
Reviewed-on: http://gerrit.causes.com/37716
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Shane da Silva <shane@causes.com>