Last tested; May 12, 2020
<=0 OR >0 OR NULL for numbers,
%,NULL for strings.
To get all non-null values, see How do I filter out null in a filtered measure?
Here are expressions that DON'T work:
NULL, NOT NULL for numbers and
NULL,-NULL for strings generate AND between the two parts, so no values will be returned.
NULL OR NOT NULL cannot be parsed as a filter expression.