I don’t think there’s a way, with GREP, to specify the beginning of a cell, unfortunately.
You can, of course, specify the beginning of a story, so if you wanted to delete all empty paragraphs at the beginning of a story (and this should include the beginning of cells), you could do that.
It would also be fairly simple to write a script to apply a certain attribute to all text in all cells (e.g. strike through, which you’re probably not using), and then to search for empty paragraphs that have that attribute applied….
The GREP to find a paragraph at the beginning of a story (including at the beginning of a cell) is
\A\r