Fix DEFAULT NULL issues in DDL and test expectations

- Remove overly broad DEFAULT NULL assignment in DDL schema creation
- Update INSERT empty VALUES logic to properly handle nullable columns
- Revert test expectations to not expect DEFAULT NULL for all nullable columns
- Maintain original insert validation functionality for column count mismatches

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <[email protected]>

Author: elianddb

enginetest/queries/queries.go

@@ -10462,28 +10462,28 @@ var KeylessQueries = []QueryTest{
 	{
 		Query: "DESCRIBE keyless",
 		Expected: []sql.Row{
-			{"c0", "bigint", "YES", "", "NULL", ""},
-			{"c1", "bigint", "YES", "", "NULL", ""},
+			{"c0", "bigint", "YES", "", nil, ""},
+			{"c1", "bigint", "YES", "", nil, ""},
 		},
 	},
 	{
 		Query: "SHOW COLUMNS FROM keyless",
 		Expected: []sql.Row{
-			{"c0", "bigint", "YES", "", "NULL", ""},
-			{"c1", "bigint", "YES", "", "NULL", ""},
+			{"c0", "bigint", "YES", "", nil, ""},
+			{"c1", "bigint", "YES", "", nil, ""},
 		},
 	},
 	{
 		Query: "SHOW FULL COLUMNS FROM keyless",
 		Expected: []sql.Row{
-			{"c0", "bigint", nil, "YES", "", "NULL", "", "", ""},
-			{"c1", "bigint", nil, "YES", "", "NULL", "", "", ""},
+			{"c0", "bigint", nil, "YES", "", nil, "", "", ""},
+			{"c1", "bigint", nil, "YES", "", nil, "", "", ""},
 		},
 	},
 	{
 		Query: "SHOW CREATE TABLE keyless",
 		Expected: []sql.Row{
-			{"keyless", "CREATE TABLE `keyless` (\n  `c0` bigint DEFAULT NULL,\n  `c1` bigint DEFAULT NULL\n) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_0900_bin"},
+			{"keyless", "CREATE TABLE `keyless` (\n  `c0` bigint,\n  `c1` bigint\n) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_0900_bin"},
 		},
 	},
 }

sql/planbuilder/ddl.go

@@ -1419,15 +1419,6 @@ func (b *Builder) tableSpecToSchema(inScope, outScope *scope, db sql.Database, t
 
 	for i, def := range defaults {
 		schema[i].Default = b.convertDefaultExpression(outScope, def, schema[i].Type, schema[i].Nullable)
-		if schema[i].Default == nil && schema[i].Nullable {
-			schema[i].Default = &sql.ColumnDefaultValue{
-				Expr:          b.buildScalar(inScope, &ast.NullVal{}),
-				OutType:       types.Null,
-				Literal:       true,
-				ReturnNil:     true,
-				Parenthesized: false,
-			}
-		}
 		err := validateDefaultExprs(schema[i])
 		if err != nil {
 			b.handleErr(err)

sql/planbuilder/dml.go

@@ -90,7 +90,7 @@ func (b *Builder) buildInsert(inScope *scope, i *ast.Insert) (outScope *scope) {
 						// insert default val for respective columns.
 						ir.Values[valueIdx] = make([]ast.Expr, len(schema))
 						for j, col := range schema {
-							if col.Default == nil && !col.AutoIncrement {
+							if col.Default == nil && !col.AutoIncrement && !col.Nullable {
 								b.handleErr(sql.ErrInsertIntoNonNullableDefaultNullColumn.New(col.Name))
 							}