Fix insertMultipleStruct to insert null value under certain circumstances (#2077)

The behavior of multi insertion of null differs from that of single insertion.
On the other hand, behavior between single and bulk insertion of null using pointer of struct field is identical.
Please see the example below.
```go
s := engineGroup.NewSession()

type User struct {
    ID   int    `xorm:"not null pk autoincr INT(10)"`
    Name string `xorm:"not null VARCHAR(191)"`
    Age  int64  `xorm:"null BIGINT(20)"`
}
list := []*User{
    {
        Name: "John",
    },
    {
    	Name: "Wick",
    },
}
s.Nullable("age")

// Single insertion works
_, err := s.Insert(list[0]) // [table] `user` INSERT INTO `user` (`name`,`age`) VALUES (?, ?) [John <nil>]

s.Nullable("age")
// Bulk insertion does not work
_, err := s.Insert(list) // [table] `user` INSERT INTO `user` (`name,`age`) VALUES (?, ?),(?, ?) [John, 0, Wick, 0]

//---------------------------------
//Using pointer, which is nullable, the generated sql has no difference.
//---------------------------------

type User struct {
    ID   int    `xorm:"not null pk autoincr INT(10)"`
    Name string `xorm:"not null VARCHAR(191)"`
    Age  *int64 `xorm:"null BIGINT(20)"`
}
list := []*User{
    {
        Name: "John",
    },
    {
    	Name: "Wick",
    },
}
s.Nullable("age")
// Single insertion works
_, err := s.Insert(list[0]) // [table] `user` INSERT INTO `user` (`name`,`age`) VALUES (?, ?) [John <nil>]

s.Nullable("age")
// Bulk insertion does not work
_, err := s.Insert(list) // [table] `user` INSERT INTO `user` (`name,`age`) VALUES (?, ?),(?, ?) [John, <nil>, Wick, <nil>]
```

Reviewed-on: https://gitea.com/xorm/xorm/pulls/2077
Co-authored-by: satorunooshie <satorunooshie@noreply.gitea.io>
Co-committed-by: satorunooshie <satorunooshie@noreply.gitea.io>

session_insert.go

@@ -142,6 +142,13 @@ func (session *Session) insertMultipleStruct(rowsSlicePtr interface{}) (int64, e
 			if len(session.statement.ColumnMap) > 0 && !session.statement.ColumnMap.Contain(col.Name) {
 				continue
 			}
+			// !satorunooshie! set fieldValue as nil when column is nullable and zero-value
+			if _, ok := getFlagForColumn(session.statement.NullableMap, col); ok {
+				if col.Nullable && utils.IsValueZero(fieldValue) {
+					var nilValue *int
+					fieldValue = reflect.ValueOf(nilValue)
+				}
+			}
 			if (col.IsCreated || col.IsUpdated) && session.statement.UseAutoTime {
 				val, t, err := session.engine.nowTime(col)
 				if err != nil {