Insert string("+") into named-range merged-cell but only if empty

So I have a cell where I'll be putting strings, but sometimes that cell will be empty, when it is I'd like to insert a "+" to indicate that the cell needs a string.

How would I go about this?

I tried referencing itself but that was dumb after thinking about it.

=IF(ARRAYFORMULA(ISBLANK(MergedCells1)) "+",)

https://docs.google.com/spreadsheets/d/1bQnGvZw7sTZ5cY1H78bywjqhOs3ceB2bpjuLsCpKklo/edit?usp=sharing