Fix Default SimpleFormIterator Push Value

[jdemangeon]

Closes #2489
Closes #3249

When we use the SimpleFormIterator inside an array of scalar values (ArrayInput) displayed as TextInput, the famous [Object object] value is displayed on "+ Add" click.

Example:

import { SimpleFormIterator } from "react-admin";

/* recipients = ["foo@bar.com", "bar@foo.com", ....] */

<ArrayInput source="recipients">
      <SimpleFormIterator>
          <TextInput />
      </SimpleFormIterator>
</ArrayInput>

Results to:

So, to avoid this, we should inject undefined instead of an empty {}.

Here is a temporary workaround for those who are having the same problem:

import React from "react";
import { SimpleFormIterator } from "react-admin";

export const SimpleFormIteratorWithUndefinedDefault = props => {
  const originPush = props.fields.push;
  props.fields.push = data =>
    originPush(JSON.stringify(data) === "{}" ? undefined : data);

  return <SimpleFormIterator {...props} />;
};

PS: This "display" is the default one used in ApiPlatform Admin for the array field type (https://github.com/api-platform/admin/blob/master/src/InputGuesser.js#L90)

[jdemangeon]

Oops, i've seen that another PR covering this issue (https://github.com/marmelab/react-admin/pull/4306/files).

[JulienMattiussi]

Unit tests ok
Manual tests ok