Steps to reproduce

1. Create a new dialog
2. Ensure that the dialog has not message key defined.
3. Open the dialog, the label key is mandatory.

Expected results

In 5.7, the key was not mandatory so the dialog title was displayed as empty when no message key was defined.

Actual results

A message key is mandatory

Workaround
N/A

Development notes

• Use newly introduced fallback to empty string

Related to MGNLUI-5814