[TYPO3-mvc] 1.4 property mapping fails with empty relations?

[Tim Schoch | GSTALTIG]

It'sa me again

The before mentioned way does not work fully. It solves the converter check, but fails to assign the value to the domain object itself.
The problem:
When the converter returns NULL, the propertyMapper skipps this property ( Line 176, if ($targetPropertyValue !== NULL) { ... )
If the converter however returns 0 or an empty string, the setter fails because it only allowes instances of the given class or NULL
> public function setOtherDomainModel(Tx_Myext_Domain_Model_Thingie $thingie = NULL) {

I for my part removed the type hint and test for it explicitly. But that can't be the way it's thought to be.
So agian, the question stands, how can a relation be made optional? Does anybody now how/if this is implemented in FLOW3?

You can get the whole code here (I updated the converter too): http://pastebin.com/yD7Vfd89

Tim