…l is not installed.

The distribution may have a `translations.json` with only an empty string.  That should not break like this:

```
Traceback (innermost last):
  Module ZPublisher.WSGIPublisher, line 181, in transaction_pubevents
  Module ZPublisher.WSGIPublisher, line 391, in publish_module
  Module ZPublisher.WSGIPublisher, line 285, in publish
  Module ZPublisher.mapply, line 98, in mapply
  Module ZPublisher.WSGIPublisher, line 68, in call_object
  Module plone.rest.service, line 21, in __call__
  Module plone.restapi.services, line 19, in render
  Module plone.distribution.services.sites.add, line 49, in reply
  Module plone.distribution.api.site, line 165, in create
  Module plone.distribution.api.site, line 104, in _create_site
  Module plone.distribution.handler, line 44, in default_handler
  Module plone.exportimport.importers, line 57, in import_site
  Module plone.exportimport.importers.base, line 70, in import_data
  Module plone.exportimport.importers.translations, line 17, in do_import
TypeError: object of type 'NoneType' has no len()
```

…onally.

Only register them when `plone.app.multilingual` and `plone.app.discussion` are installed.
Prevent error when an exporter does not actually export any paths, but returns None, like before I made these two conditional:

```
$ bin/export-distribution parts/zeoclient-classic/etc/zope.conf Plone
Exporting Plone site at /Plone
 Target path: /Users/maurits/community/plone-coredev/6.1/src/plone.classicui/src/plone/classicui/distributions/classic/content
Traceback (most recent call last):
  File "/Users/maurits/community/plone-coredev/6.1/bin/export-distribution", line 250, in <module>
    sys.exit(plone.distribution.cli.export())
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/maurits/community/plone-coredev/6.1/src/plone.distribution/src/plone/distribution/cli/__init__.py", line 44, in export
    results = get_exporter(site).export_site(path)
              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/maurits/community/plone-coredev/6.1/src/plone.exportimport/src/plone/exportimport/exporters/__init__.py", line 72, in export_site
    paths.extend(exporter.export_data(path))
TypeError: 'NoneType' object is not iterable
```

Set it to neutral None, unless it is explicitly set.

`json.dump` does not add this, so we explicitly do it.
Otherwise when you manually edit a file and you use an editor that respects the standard `.editorconfig` that we have in most Plone packages, you always get a diff because there is a newline at the end.

We now have some data with a blacklist_status, and this gave an error.

We still read the old key for backwards compatibility.