This is certainly not the "best" solution but it may work. On a dashboard, When a sheet is empty, it is actually Transparent.
If your entire Dashboard is floated, You can Float your Sheet above some Text. Let the Text be your desired "Title when blank". When the sheet is blank, that "Title" will show.
However, I haven't been able to make it work with two titles... Hopefully this steers you in the right direction!
Thank you. It's a smart workaround.