Function nodes fall back to self when its function owner becomes null/invalid

[jdaignault]

It is possible for a function node to fall back to self as its Function Owner when it contains an error.

Steps to reproduce:

1. Create a widget with two TextBlock, and ensure that both are exposed as variables
2. Create a binding with a Read a Property node for the first TextBlock
3. Connect its output to a Call a Function destination node, and set its function to Set Visibility with InVisibility parameter set to Collapsed
4. Switch to Designer mode, and delete the first TextBlock
5. Observe that the Call a Function node in the Binding Editor reports an error
6. Add the widget to the HUD and run PIE
7. Observe that the widget is not displayed at all. Note that instead of not executing the binding at all, the owning widget’s visibility has been set to Collapsed
8. Delete the binding and run PIE again
9. Observe that the widget is now visible