[RFR] Add emptyText prop to show a fixed string when the value of a field is null

[m4theushw]

Closes #2544

TODO:
- Fix docs
- Merge against next

[fzaninotto]

Nice!

Don't forget to update the documentation about common field props, too.

[fzaninotto]

why did you change that?

[m4theushw]

The recompose/pure in EmailField was not playing well with enzyme.

[fzaninotto]

That's great!

I have a general concern about how the emptyText renders in your PR. In some components, you put it inside another element (e.g. ChipField), in others, inside a span with the className, and in some others, you just render the emptyText directly.

I think all the components should behave the same. There are 2 options:

1. return the emptyText without formatting by default (letting the developer pass an element wrapped inside Typography to let them style the empty text)
2. return the emptyText formatted just like a regular content

I think 2. is preferable (because of the principle of least WTF). What's your opinion?

[m4theushw]

Ok, I'll standardize all components to return a Typography with emptyText inside. Maybe in the future depending on the acceptance by the public we can allow to override by passing an element.

[fzaninotto]

If the developer wants a custom component rendered in case of null value, they can still wrap the Field inside a component of their own.

[fzaninotto]

Thanks!