[SIEM] Maps adds incorrect filter to KQL bar when filtering on property existence

When using the `Filter on property` functionality within tooltips in the map, if the property doesn't exist, the KQL filter is added as 

```
source.domain: "undefined"
``` 

instead of 

```
(NOT source.domain:*)
```

![maps_filter_undefined](https://user-images.githubusercontent.com/2946766/64207595-2be02880-ce5a-11e9-96ba-f7e1f182731f.gif)
