Add null-equals-null JOIN support in Substrait producer/consumer

[nseekhao]

Is your feature request related to a problem or challenge? Please describe what you are trying to do.
DataFusion Substrait producer/consumer currently does not support JOINs that treat a NULL key as the same value as another NULL key. This is needed in cases such as INTERSECT, which uses LeftSemi Join to compute the key intersection. In this case, a NULL key from the left should be treated as equal to a NULL key from the right.

Describe the solution you'd like
Support for null-equals-null joins

Describe alternatives you've considered
N/A

Additional context
N/A