feat: Add placeholder prop to <SelectArrayInput>

[erwanMarmelab]

Closes #10454.

Problem

<SelectArrayInput> cannot render a custom text when no choice is selected (e.g. "All Channels" in a filter).

Solution

Add a placeholder prop, rendered in place of the chips while the value is empty.

How To Test

http://localhost:9010/?path=/story/ra-ui-materialui-input-selectarrayinput--placeholder

Additional Checks

• The PR targets master for a bugfix or a documentation fix, or next for a feature
• The PR includes unit tests (if not possible, describe why)
• The PR includes one or several stories (if not possible, describe why)
• The documentation is up to date

Also, please make sure to read the contributing guidelines.

[fzaninotto]

I think this PR adds another feature than the one requested in #10454.

• emptyText, as used in SelectInput and AutocompleteInput, adds an option in the dropdown for the empty value
• placeholder, as you implemented it, renders an element in the input when nothing is selected.

By the way the story you added is backwards: the placeholder says "all selected" when in reality it should be "none"