Add an empty option to g:select and the associated controller logic

When an object from a class Foo is associated with a Bar object, we don't always want to have the association.
So when in the view we've got a select which shows all Bar instances, an empty option would be good to.

Example use case from Fred Janon:

I have an "Shift" domain class that references an "Officer" domain
class. An "Officer" can be optional assigned to a "Shift". The user can
change the date/time of the "Shift" and at some point assign an
"Officer" to the shift. By default Grails builds an "Edit" view for the
"Shift" with a "Select" referencing the "Officer" list. The problem is
that the "Select" doesn't have an empty entry for the "Officer". Even
if the user wants to change only the time of the shift, the first
officer will be selected by default and saved.

Is there a way to make the "Officer" field optional in the "Edit Shift"
view?

This is the code generate by Grails:

<g:select optionKey="id" from="$

"
name="officer.id" value="$

"></g:select>

Any suggestion to do this cleanly?