Fix empty line after adding an entry to the Custom Properties

Steps to reproduce:

1. Go to Modeler > Content Model > Document Types and create a new document type
2. On the newly created document type, edit the Title widget
3. Go to the "Custom properties configuration" block and add an entry
4. Save the widget
5. Edit the same widget
6. Observe that a new empty line has been added to the "Custom properties configuration" block
7. Save the widget => An error is displayed in a popup : Invalid value: Invalid empty key on property 'Custom properties'

Expected behavior : no empty line is added to the "Custom properties configuration" block and it's possible to save the widget without making change to the custom properties