Fix input merge labels input node combo box.
Fix input merge labels input node combo box.

The input was not correctly updated with the node of the label map
collapsible group.

See Issue #13882

finetjul/bender,ricortiz/BenderCraniosynostosis,vovythevov/Bender,ricortiz/Bender,ricortiz/BenderCraniosynostosis,vovythevov/Bender,ricortiz/Bender,finetjul/bender,vovythevov/Bender,ricortiz/BenderCraniosynostosis,ricortiz/Bender,ricortiz/Bender,finetjul/bender,finetjul/bender,ricortiz/BenderCraniosynostosis,vovythevov/Bender