In this case, I gave the resource update a title.
I later chose to edit the resource update title to be empty. When you edit a resource update, the title is said to be optional (suggesting it could be empty).
I realise "Optional" refers to the edit - i.e. it isn't necessary to change this, but it suggests it could be empty.
This time, I chose not to use a title (so I removed it). I saved this update, and it told me that a title is required.