How to set parameter to null in a web service call

This morning we had an interesting question wit Martin about to send a null parameter in a web service call.

For example in the update_user function, we could have a field:
'age' => new external_value(PARAM_INT, 'User age', VALUE_OPTIONAL),
Imagine that the ws client wants to set this field to null. (because he entered a wrong age and finally he doesn't really know what is the user age.)

In this case a solution could be:
the ws client sends a '' or 'null' value. Then the external update_user function compare this value, and replace by php null, before to call the validate_parameters function.