Added filter for empty rows in Define New Filter dialog

[shadrackkibet]

fixes #7617

@rdstern I added an option "isEmpty" into the dropdown. When you select this option it allows you to click Add Condition then you can select only empty rows. What do you think?

[rdstern]

That's an interesting way to do it - and seems to work fine. I thought we would just enable the As Condition button with an empty string, but I am also happy with this solution.
a) I assume we can name it as we want, so would is.empty, rather than isEmpty be more consistent with the is.na?
b) We should then also add !is.empty for the opposite - as we have for !is.na.

[shadrackkibet]

@rdstern I agree to the request above and I have actioned it accordingly.

[rdstern]

@shadrackkibet this is great. Not the way I was thinking, but this way seems better. And excellent that you are back to R-Instat, even though no longer with INNODEMS. I hope @lloyddewit can approve, so you can proceed to the useful ADD facility after this is merged!

[lloyddewit]

@shadrackkibet Tidy fix, thanks, happy to approve.
I just fixed a small indentation issue.