Insert an active model and get back the last insert id doesn't work with autoincrement

[Razzwan]

Description

Insert an active model and get back the last insert id doesn't work with autoincrement id.

Steps to Reproduce

1. Create migration and generate a model

   #[derive(Clone, Debug, PartialEq, DeriveEntityModel, Eq)]
   #[sea_orm(table_name = "fruit")]
   pub struct Model {
   #[sea_orm(primary_key)]
   pub id: i32,
   }

2. Insert an active model and get back the last insert id using static method

   
   let pear = fruit::ActiveModel {
   ..Default::default() // just only autoincremented id here
   };

let res: InsertResult = Fruit::insert(pear).exec(db).await?;

### Expected Behavior

It must insert and return saved id

### Actual Behavior

It leads an error:

"Custom Error: Custom Error: Attribute id is NotSet"

### Reproduces How Often

All the time

### Workarounds

Don't use static method and use such one instead:
```rust
let pear = fruit::ActiveModel {
    ..Default::default()
};

let pear: fruit::Model = pear.insert(db).await?;

Reproducible Example

1. Run migration:

   
   use sea_orm_migration::prelude::*;

[derive(DeriveMigrationName)]

pub struct Migration;

[async_trait::async_trait]

impl MigrationTrait for Migration { async fn up(&self, manager: &SchemaManager) -> Result<(), DbErr> { manager .create_table( Table::create() .table(Fruit::Table) .if_not_exists() .col( ColumnDef::new(Fruit::Id) .integer() .not_null() .auto_increment() .primary_key(), ) .to_owned(), ) .await }

async fn down(&self, manager: &SchemaManager) -> Result<(), DbErr> {
    manager
        .drop_table(Table::drop().table(Fruit::Table).to_owned())
        .await
}

}

[derive(DeriveIden)]

enum Fruit { Table, Id, }

3. Generate a model based on migration above

3. Then try to insert entity using generated model:
```rust
let pear = fruit::ActiveModel {
    ..Default::default() // just only autoincremented id here
};

let res: InsertResult = Fruit::insert(pear).exec(db).await?;

Versions

0.12.14