ENH Show a cannot create message when types is empty

[emteknetnz]

Issue #151

Test with

    public function getCMSFields()
    {
        // ...
        LinkField('MyLink')::create()->setAllowedTypes([MyLink::class]));
        // ...

Where MyLink has

    public function canCreate($member = null, $context = [])
    {
        return false;
    }

[sabina-talipova]

I've tested locally, works perfect. Looks good, just small question.

[sabina-talipova]

LGTM.