If a repeat has no label, improve the add repeat dialog text

[lognaturel]

https://forum.getodk.org/t/repeat-groups-add-null-repeat-dialogue-and-remove-repeat/49836

Currently if a repeat has no label, you get a message like "Add "null"" as the prompt to add a new repeat. Let's at least change that to "Add another?" and consider spending a bit of design time on this dialog soon.