Below is how to display cells with missing information inside a cell named missing data.
Example Database
Formula
if(empty(prop("Column 1")), "⌫ Column 1\n", "") + if(empty(prop("Column 2")), "⌫ Column 2\n", "") + if(empty(prop("Column 3")), "⌫ Column 3\n", "") + if(empty(prop("Column 4")), "⌫ Column 4\n", "") + if(empty(prop("Column 5")), "⌫ Column 5\n", "") + if(empty(prop("Column 6")), "⌫ Column 6\n", "")