[transform-modules-systemjs] top level this not mapped to undefined

[LarsDenBakker]

Bug Report

Current Behavior
When using @babel/plugin-transform-modules-systemjs the top level this is not mapped to undefined.

When using @babel/plugin-transform-modules-commonjs it is mapped correctly to void 0.

Reproduction
SystemJS (incorrect):
https://babeljs.io/repl#?babili=false&browsers=chrome%2078&build=&builtIns=false&spec=false&loose=false&code_lz=BQMwrgdgxgLglgewgAmAJwQmBKZBvAKGWSiQGcEAbAUwDpKEBzYAcgywC5kWAaZdnAG4CAX2zAYACzhlkAHznIA7nAgATBEuyCgA&debug=false&forceAllTransforms=false&shippedProposals=true&circleciRepo=&evaluate=false&fileSize=false&timeTravel=false&sourceType=module&lineWrap=false&presets=&prettier=false&targets=&version=7.7.4&externalPlugins=%40babel%2Fplugin-transform-modules-systemjs%407.7.4

CommonJS (correct): https://babeljs.io/repl#?babili=false&browsers=chrome%2078&build=&builtIns=false&spec=false&loose=false&code_lz=BQMwrgdgxgLglgewgAmAJwQmBKZBvAKGWSiQGcEAbAUwDpKEBzYAcgywC5kWAaZdnAG4CAX2zAYACzhlkAHznIA7nAgATBEuyCgA&debug=false&forceAllTransforms=false&shippedProposals=true&circleciRepo=&evaluate=false&fileSize=false&timeTravel=false&sourceType=module&lineWrap=false&presets=&prettier=false&targets=&version=7.7.4&externalPlugins=%40babel%2Fplugin-transform-modules-commonjs%407.7.4

Expected behavior/code

System.register([], function (_export, _context) {
  "use strict";

  return {
    setters: [],
    execute: function () {
      (function (root) {
        console.log('root: ', root);
      })(void 0 || window);
    }
  };
});

[babel-bot]

Hey @LarsDenBakker! We really appreciate you taking the time to report an issue. The collaborators on this project attempt to help as many people as possible, but we're a limited number of volunteers, so it's possible this won't be addressed swiftly.

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