ddl:fix type bit could have null as its default value (#7604)

* ddl:fix type bit could have null as its default value

ddl/db_test.go

@@ -1534,6 +1534,47 @@ func (s *testDBSuite) TestBitDefaultValue(c *C) {
 	tk.MustExec("insert into t_bit set c2=1;")
 	tk.MustQuery("select bin(c1),c2 from t_bit").Check(testkit.Rows("11111010 1"))
 	tk.MustExec("drop table t_bit")
+	tk.MustExec(`create table testalltypes1 (
+    field_1 bit default 1,
+    field_2 tinyint null default null
+	);`)
+	tk.MustExec(`create table testalltypes2 (
+    field_1 bit null default null,
+    field_2 tinyint null default null,
+    field_3 tinyint unsigned null default null,
+    field_4 bigint null default null,
+    field_5 bigint unsigned null default null,
+    field_6 mediumblob null default null,
+    field_7 longblob null default null,
+    field_8 blob null default null,
+    field_9 tinyblob null default null,
+    field_10 varbinary(255) null default null,
+    field_11 binary(255) null default null,
+    field_12 mediumtext null default null,
+    field_13 longtext null default null,
+    field_14 text null default null,
+    field_15 tinytext null default null,
+    field_16 char(255) null default null,
+    field_17 numeric null default null,
+    field_18 decimal null default null,
+    field_19 integer null default null,
+    field_20 integer unsigned null default null,
+    field_21 int null default null,
+    field_22 int unsigned null default null,
+    field_23 mediumint null default null,
+    field_24 mediumint unsigned null default null,
+    field_25 smallint null default null,
+    field_26 smallint unsigned null default null,
+    field_27 float null default null,
+    field_28 double null default null,
+    field_29 double precision null default null,
+    field_30 real null default null,
+    field_31 varchar(255) null default null,
+    field_32 date null default null,
+    field_33 time null default null,
+    field_34 datetime null default null,
+    field_35 timestamp null default null
+	);`)
 }
 
 func (s *testDBSuite) TestCreateTableWithPartition(c *C) {

model/model.go

@@ -96,6 +96,10 @@ func (c *ColumnInfo) SetDefaultValue(value interface{}) error {
 	if c.Tp == mysql.TypeBit {
 		// For mysql.TypeBit type, the default value storage format must be a string.
 		// Other value such as int must convert to string format first.
+		// The mysql.TypeBit type supports the null default value.
+		if value == nil {
+			return nil
+		}
 		if v, ok := value.(string); ok {
 			c.DefaultValueBit = []byte(v)
 			return nil