Your filter should work correctly now if you use gdead as the data source for the gallery. FYI: I wrote it as a collection in case you did not want to modify the original data source. If that is not a problem you can run the update on the original data source to convert the text items to 0.
For example I created list "Example". There is a column "Number And/or Text" with type - single line of text. There I have 7 Items with numbers, empty, "N/A" and "null". On screenshot you see gallery and formula at the top for label: