Multiple Navigational Properties to the same entity with composite primary key

### Bug Report

I have two entities `EntA` & `EntB` defined as follows:

```
id1 = $id1;
$this->id2 = $id2;
}
}

#[ORM\Entity]
class EntB {
#[ORM\Id]
#[ORM\Column(length=50)]
private string $id;
#[ORM\ManyToOne(targetEntity: EntA::class)]
#[ORM\JoinColumn(name: "ref_id", referencedColumnName="id1")]
#[ORM\JoinColumn(name: "ref_id2", referencedColumnName="id2")]
private ?EntA $ref1 = null;
#[ORM\ManyToOne(targetEntity: EntA::class)]
#[ORM\JoinColumn(name: "ref_id", referencedColumnName="id1")]
#[ORM\JoinColumn(name: "ref_id2", referencedColumnName="id2")]
private ?EntA $ref2 = null;
#[ORM\ManyToOne(targetEntity: EntA::class)]
#[ORM\JoinColumn(name: "ref_id", referencedColumnName="id1")]
#[ORM\JoinColumn(name: "ref_id3", referencedColumnName="id2", nullable=true)]
private ?EntA $ref3 = null;

public function __construct(EntA $ref1, EntA $ref2, ?EntA $ref3 = null) {
$this->id = random_str();
$this->ref1 = $ref1;
$this->ref2 = $ref2;
$this->ref3 = $ref3;
}
}
```

The design is to ensure that all reference to `EntA` from `EntB` all belong under the same parent and by parent, I mean `EntA::$id1`.

| Q | A
|------------ | ------
| BC Break | no
| Version | 2.14.0

#### Summary

Whenever I instantiate `EntB` and set `EntB::$ref3` to null, doctrine sets the column `ref_id` in the `EntB` table to null thereby making the other `EntA` references to be orphans. even though columns `ref_id1`, `ref_id2` have their values intact while column `ref_id3` is null as expected.

#### Current behavior
```
EntA Table
id1: ABC, id2: 123
id1: ABC, id2: 234

EntB Table
id: aystetx, ref_id: null, ref_id1: 123, ref_id2: 234, ref_id3: null
```
#### How to reproduce
```
$ent_a_1 = new EntA("ABC", "123");
$entityManager->persist($ent_a_1);
$ent_a_2 = new EntA("ABC", "234");
$entityManager->persist($ent_a_2);

$ent_b = new EntB($ref_a_1, $ref_a_2);
$entityManager->persist($ent_b);

$entityManager->flush();
```
#### Expected behavior
```
EntA Table
id1: ABC, id2: 123
id1: ABC, id2: 234

EntB Table
id: aystetx, ref_id: ABC, ref_id1: 123, ref_id2: 234, ref_id3: null
```

Start by reproducing the EntA and EntB mappings from the issue on Doctrine ORM 2.14.0 and inspect the SQL or persisted row after flush. Trace how multiple composite-key associations are synchronized when ref3 is null. Done means ref_id remains populated for ref1 and ref2 while only the nullable ref3 column is null.

Written by the indexing model from the issue text.