I know that to join the contents of several cells into a single one
(concatenate is the technical word), all it takes is the function & and that
to have values separated by a comma and a space all you have to do is:
a1&", "&a2&", "&a3&", "&a4

My problem is that sometimes some of these cells are empty so I end up with
some values separated by commas, empty spaces and more commas without more
values in between.

What would be the way to concatenate only the non-empty cells?

Thanks