[babel] Fix default value for `environment` option

When an `environment` isn't explicitly provided by the user's config, we used to default this to `null` in `parsePluginOptions` which is called right at the start of the Babel plugin. These parsed options were then being passed to zod, which was expecting an object type, not null. Zod can reify a default config based on the schema, if an empty object is passed in. So by changing our default value to `{}` this should fix the compiler bailing out incorrectly on valid compiler options Test plan: tested this manually on the external repo by modifying node_modules