Example Map

See layer "Peaks", Filter "elevation"

Steps to Reproduce:

1. Create a layer
2. Add a number of markers
3. Define a custom numeric field ("elevation" in this example)
4. Define values on some markers but leave some unset
5. Define a map filter for the numeric field, select type either as Exclusive selection or Multiple selection
6. Select a filtered value of ""
7. Behavior as expected: only markers with an unset value ("elevation" in this example) are shown
8. Select a numeric value
9. Expected Behavior: only markers with that value in the field are shown
10. Actual Behavior: No markers from that layer are shown