Fix EZP-21388: Object relation fails to be selected with eZ Find search

**eZContentObjectTreeNode** constructor doesn't assign **ContentObjectID** value unless it's already in cache. 
Therefore, **eZFindResultNode** was not properly prepared to be used in browse selection templates.

This approach might be a bit overkill, for the problem at hand, but the other solution I could foresee would implicate messing with eZContentObjectTreeNode itself. That could induce much more load on the system.
 
Manual tested
Note: *Since content objects tend to become cached through the testing steps, the patch seemed useless after a while. But after the test environment was restarted, the original bug was always present without the patch, and never reproducible with it applied*