Is there a NULL value for a multi-value list?
Select Element's API

#empty_option: (optional) The label to show for the first default option. By default, the label is automatically set to &quot;- Select -&quot; for
a required field and &quot;- None -&quot; for an optional field.
#empty_value: (optional) The value for the first default option, which is used to determine whether the user submitted a value or not.

So using this information, you are able to use the null value's defaults from the select element.