Add support for empty in Reference fields

[fzaninotto]

Problem

When using Reference fields (<ReferenceField>, <ReferenceOneField>, <ReferenceManyField>), to define a custom view for the empty case, developers have to create a custom child that reads the list context. This is super cumbersome.

Solution

Add support for the empty prop.

<ReferenceManyField
  reference="books"
  target="author_id"
  empty="no books"
>
    ...
</ReferenceManyField>

Additional Checks

• Extract <ReferenceOneFieldBase> from <ReferenceOneField>
• Extract <ReferenceManyFieldBase> from <ReferenceManyField>
• Add <ReferenceField empty> prop
• Add <ReferenceArrayField empty> prop
• Add <ReferenceManyField empty> prop
• Add <WithRecord empty> prop
• Update doc
• Add tests

The empty logic is in ra-core, so it can be reused for other UI kits.

The changes are backward compatible: emptyText is still supported where it was implemented, but it's deprecated.

[djhi]

It's nice that we have more base components! 💯 That would be even nicer with stories and tests for those :)

[fzaninotto]

It's nice that we have more base components! 💯 That would be even nicer with stories and tests for those :)

Done is better than perfect ;)

The base components are tested indirectly via MUI components.