planner: add note for typed null projection

Author: lilinghai

pkg/planner/core/rule/rule_outer_join_to_semi_join.go

@@ -385,6 +385,9 @@ func generateProjectForConvertAntiJoin(p *logicalop.LogicalJoin, innerSchSet *in
 	projExprs := make([]expression.Expression, 0, len(parentNodeSchema.Columns))
 	for _, c := range parentNodeSchema.Columns {
 		if innerSchSet.Has(int(c.UniqueID)) {
+			// Keep null-extended inner columns aligned with the parent schema type.
+			// NewNull() defaults to TINYINT, while the original column can have
+			// another type and must become nullable after the outer join rewrite.
 			retType := c.RetType.Clone()
 			retType.DelFlag(mysql.NotNullFlag)
 			projExprs = append(projExprs, expression.NewNullWithFieldType(retType))