[NGSIMongoSink, NGSISTHSink] Fix collection building when data_model=dm-by-service-path

[frbattid]

case DMBYSERVICEPATH:
                if (fiwareServicePath.equals("/")) {
                    throw new CygnusBadConfiguration("Default service path '/' cannot be used with "
                            + "dm-by-service-path data model");
                } // if

                collectionName = NGSIUtils.encodeSTHCollection(fiwareServicePath);
                break;

The if part is not necessary.

[frbattid]

Fixed in PR #979