Incorrect results (including nulls) when querying string column with col <> '' and col is not null

### Affected Version
0.18.0 and 0.20.0

### Description
For druid 0.18.0 and config useDefaultValueForNull to false, querying a table with condition on string column (lets say col) with condition like col <> '' and col is not null returns rows with that column null. Both of these conditions behave correctly if passed separately but when together, nulls are not filtered at all.
- Cluster size
Local docker setup
- Configurations in use
useDefaultValueForNull is False
- Steps to reproduce the problem
1. Setup local druid 0.18.0 using docker setup
2. Load example data viz. wikipedia
3. Query: select * from wikipedia where cityName is not null and cityName <> '' limit 100
- The error message or stack traces encountered.
In the results you can see rows with cityName null as well
- Any debugging that you have already done
I tested it with version 0.18.0 and 0.20.0 and both have these issue

Reproduce the issue using the local Docker setup, the stated useDefaultValueForNull=false configuration, and the Wikipedia query with both predicates. Trace how the combined `cityName is not null` and `cityName <> ''` conditions are evaluated, then verify that the result contains no null cityName rows in both affected versions.

Written by the indexing model from the issue text.