Fix bug when clearing a relationship which do not have an inverse

If the inverse of a relationship does not exist, the code in `clearUnmaterializedHasMany` will throw an error : `Uncaught TypeError: Cannot read property 'name' of null`

This commit fix it by checking the existence of the inverse before trying to clear the associated objects.

I wanted to write a failing test for this but it looks like the code for clearing relationship is not covered by any test.. or did I just fail to find it ?