When using the feature 'Open in Excel' for grid results, I'm unable to determine in Excel which database values were empty strings vs. NULL values. I like that it no longer exports the word 'NULL', but I'd like some sort of option to be able to make a distinction between the two.

My idea is to use a cell fill pattern (Format Cells + Fill + Pattern Style), but I would be satisfied with just about anything to be able to make that distinction. The pattern I recommend is the most sparsely dotted option, which shows a tool-tip of 6.25% Gray. That provides both a visual cue and something I can filter on. The filter option shows up for me under "Filter by Color", assuming that the column has more than one pattern in it.

Edited to add:
This might be helped or made possible by making a slight, hidden, technical change in how the data is exported to Excel. I noticed that when I run the Excel formula =ISBLANK() on cells that appear blank (in the data set exported by "Open in Excel"), the function returns FALSE. If NULL values could be exported to Excel in a way that allows ISBLANK() to return TRUE, then either you or we could have the option to format blank cells in the data range, allowing us to see the difference between a database NULL vs empty string (''). One way to do that is with VBA code, doing something like the following, where MyRange is an Excel range object of the exported dataset:
MyRange.SpecialCells(xlCellTypeBlanks).Interior.Pattern = xlPatternGray8