dont update record field if session blank

Good morning,
I am trying to figure out how to acheive the following.

I have billing and shipping address sessions set during checkout.
After checkout, I would like to update the user record with session values.
This I have acheived.

However, if during checkout the billing address is left blank (therefore session is blank), when the update runs, it blanks out the billing address record. (as you'd expect.)

Is there a way to use the update behaviour, so that if the session value is set, but blank, the update will not overwrite what is already held in the database field?

It's got my mind bending. And I can't think of a neat solution.
Any help would be appreciated.

Thanks,
Ben

I had a brainwave, but if you could verify it would work and help with syntax, that would be useful.

I think I can work around my problem if I have two update behaviours on same page.
If I can 'trigger' the update behaviours using a session value (for example, sessionBillingEmpty=true/false) I can remove the bindings on one of the updates, so a blank is not put in.

Question is, can I trigger the update behaviour based on a session value?

Thanks,

Ben

send a copy of the page please

Heres the update page, but with only one update behaviour. And currently the trigger is something different.
I am setting a session variable earlier on in the checkout process called 'checkoutUpdateBilling', which will be set to true or false.

Let me know if I'm barking up the wrong tree.
I realise now, I can just make two update pages and use session to manipulate link on prior page.
But if there is an easy way with trigger, that would be a useful tool.

Ben

I'm not sure i entirely understand the problem, see PM