3.5.202升级3.5.203后mysql下实体无主键时自动迁移尝试删除主键报错

``` C#
[Table(Name = "t_demo")]
[Index("{TableName}_idx", "A,B", IsUnique = true)]
public class DemoEntity
{
public Guid A { get; set; }
public Guid B { get; set; }
}

var freeSql = new FreeSqlBuilder()
.UseConnectionString(DataType.MySql, "")
.Build();
var sql = freeSql.CodeFirst.GetComparisonDDLStatements(typeof(DemoEntity));
if (!string.IsNullOrWhiteSpace(sql))
{
Console.WriteLine(sql);
freeSql.CodeFirst.SyncStructure(typeof(DemoEntity));
}

Console.WriteLine("OK");

```
3.5.202及之前没有问题，从3.5.203及之后，第一次执行上面代码自动迁移没问题，再次执行就会报错，尝试去删除主键，但是没有设置过主键。