Pulldown list skips "None" when only one value is available (solved)

We have added a pulldown list for an optional second NIC in a VM Provisioning dialog.
When just one network is available, the network is preselected. The pulldown list entry “None” should be visible when entering the form, but the one available network is shown in stead. When multiple networks are available there is no problem and “None” is visible.

We use the following dialog definition for the pulldown list of the optional second NIC:

        :dvs: true
        :vlans: true
      :method: :custom_allowed_vlans
    :description: NIC 2 - vLan
    :required: false
    :display: :edit
    :data_type: :string

Is there any documentation that clarifies the dialog definition language?

Please help to find the correct settings to prevent preselecting the one available option for a non-required pulldown list.

Try adding :auto_select_single: false to the field definition.

Thanks, that works!
Where did you find this. Is this documented somewhere?