Possible unhandled null value in `ExpressionSelectors`

[xanec]

https://github.com/druid-io/druid/blob/f18cc5df97e5826c2dd8ffafba9fcb69d10a4d44/processing/src/main/java/io/druid/segment/virtual/ExpressionSelectors.java#L108-L112

The above call baseSelector.get().asDouble() in ExpressionSelectors.makeDoubleColumnSelector may result in NullPointerException if the returned ExprEval contains null value.

Note that in comparison, a similar call in ExpressionSelectors.makeFloatColumnSelector is handled as follows:

https://github.com/druid-io/druid/blob/f18cc5df97e5826c2dd8ffafba9fcb69d10a4d44/processing/src/main/java/io/druid/segment/virtual/ExpressionSelectors.java#L83-L87

I believe this is introduced in #4491 (@b-slim)

[gianm]

Yes, this code is wrong. It should be more like what makeFloatColumnSelector does.

[gianm]

Thanks for the catch @xanec ! Would you be interested in doing a patch to fix it?

[xanec]

Sure!
Note: #4669