Add menu item placeholder for TaxonomyPart and TaxonomyFieldSettings

[MikeAlhayek]

When adding a new Taxonomy, the Term Content Type by default selected the very first content type that is available in the menu because there is no placeholder.

This happens because we do not have an option with no value.

By default we should have a placeholder so that the use knows that they need to select a value. Also, the Term Content type should be required field and Title Part should be required by default.

Also, in the TaxonomyFieldSettings, we have the same missing-placeholder problem.

It should look like this