<AutocompleteInput>do not support to be empty

[anewcoder1992]

What you were expecting:
allow empty of the AutocompleteInput

What happened instead:
AutocompleteInput after select one value ,even you clear the input , always save the last selected choice .

Steps to reproduce:

1. https://codesandbox.io/s/simple-ucjul
   2.enter to create comment page
   3.choose one item of posts,
   4.clear your input and leave
   5.you can see the input will load the value selected last time
   Related code:

    <ReferenceInput {...props} defaultValue={newPostId}>
                <AutocompleteInput optionText="title" allowEmpty />
            </ReferenceInput>

In document we can see following :

allowEmpty | Optional | boolean | false | If false and the searchText typed did not match any suggestion, the searchText will revert to the current value when the field is blurred. If true and the searchText is set to '' then the field will set the input value to null. |
but after selected one when the field is blurred. the input never became empty.
Environment

• React-admin version: next

[MiMo42]

Yes. see #3920
The empty item is missing as soon as a suggestion has been selected.

[Kmaschta]

Closing as duplicated as #3920