Type: mixed
Possible values: true, false, or a string containing a label for the null value

If set to true, this parameter sets the first option of a select field to a null (blank) value.

A string can also be passed in to specify a label for the blank value:

"allow_null" => "select a category :";

The above code would create a select menu with “select a category: ” as the default option with no value assigned, like so:

This parameter may only be used with select fields.