Filtering for blank values is akward at the moment

[emmanuelle]

Triggerered by this community post.

In the context of editable tables, filtering for blank cells makes a lot of sense.

In order to filter for blank (non-defined) values, at the moment one has to do something like {col} is nil or {col} == ''. It would be more readable to have a syntax like is blank (or something to be discussed).