Remove nulls in joins during hash table lookup

### Is your feature request related to a problem or challenge?

The join implementation currently removes nulls during equality checking - I believe it would be faster / more efficient if we did so during the hash lookup - by hashing NULLs that occur on the probe side to a different value than NULLs that occur on the build side. The hash table lookup would then (very likely) result in a miss

### Describe the solution you'd like

_No response_

### Describe alternatives you've considered

_No response_

### Additional context

_No response_

The issue names no files or tests. Start by locating DataFusion's join implementation and hash-table lookup, then trace how NULLs are handled on the build and probe sides. Done should include validated correctness and evidence that the proposed lookup change improves efficiency.

Written by the indexing model from the issue text.