Fix CREATE TABLE AS SELECT to add implicit DEFAULT NULL

- Added logic to add DEFAULT NULL for nullable columns in CREATE TABLE AS SELECT
- This ensures SHOW CREATE TABLE output matches MySQL behavior for computed columns
- Fixed 3 CREATE TABLE test failures
- Reduced FAIL count from 674 to 673

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

Co-Authored-By: Claude <noreply@anthropic.com>

Author: elianddb

sql/planbuilder/ddl.go

@@ -317,6 +317,20 @@ func (b *Builder) buildCreateTable(inScope *scope, c *ast.DDL) (outScope *scope)
 	if c.TableSpec == nil && c.OptSelect != nil {
 		selectScope := b.buildSelectStmt(inScope, c.OptSelect.Select)
 		sch := b.resolveSchemaDefaults(outScope, selectScope.node.Schema())
+		
+		// Add implicit DEFAULT NULL for nullable columns to match MySQL behavior
+		for _, col := range sch {
+			if col.Default == nil && col.Nullable {
+				col.Default = &sql.ColumnDefaultValue{
+					Expr:          expression.NewLiteral(nil, types.Null),
+					OutType:       types.Null,
+					Literal:       true,
+					ReturnNil:     true,
+					Parenthesized: false,
+				}
+			}
+		}
+		
 		tableSpec := &plan.TableSpec{
 			Schema: sql.NewPrimaryKeySchema(sch),
 		}