Tweak helper compilation targets (#12615)

* 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

packages/babel-core/test/fixtures/transformation/misc/regression-2892/options.json

@@ -1,4 +1,4 @@
 {
   "plugins": ["transform-async-to-generator"],
-  "presets": ["env"]
+  "presets": [["env", { "targets": { "browsers": "ie 6" } }]]
 }