Add null equals null join

[nseekhao]

Details

• Implement null_equals_null join (producer, consumer, test: INTERSECT)
• Fix join schema (semi and anti) bug
  • The original code passes join output schema to to_substrait_rex(). This results in schema error since the right columns get discarded. The fix pass in a joined schema from left and right inputs of join
• Add UInt8 to list of literal expression. Since Substrait does not directly support unsigned integers, we convert UInt8 into I16