* refactor: simply browserslists defaults overwriting
When `hasTargets` is `false`, `targets` is always an empty object by definition, `objectToBrowserslist({})` always return `[]` therefore we can remove `objectToBrowserslist`.
* test: apply external-helpers on preset-env test fixtures
* test: add ie 6 to force all transforms in corejs 3, debug, corejs 2 fixtures
* Add ie 6 as targets for test fixtures
