Mongo ODM Fields cannot be unset

**API Platform version(s) affected**: 2.5.7

**Description**
Treatment of `Doctrine\ODM\MongoDB\Mapping\Annotations\Field`'s `nullable` property is incorrect.

In MongoDB ODM's mapping info, `nullable` doesn't mean "is the field allowed to be null" in the database/php etc like it does in ORM. All fields in mongodb are nullable, in ODM it means when mapping from PHP does null map to an absent key in the database document or not.

https://www.doctrine-project.org/projects/doctrine-mongodb-odm/en/2.1/reference/annotations-reference.html#field

e.g. `nullable=false` (Default)

```php
$document->setField(null);
$dm->flush();
```

```json
{
"_id": ObjectId("***")
}
```

e.g. `nullable=true`

```php
$document->setField(null);
$dm->flush();
```

```json
{
"_id": ObjectId("***"),
"field": null
}
```

**How to reproduce**

Given the following field on an ODM document:

```php
use Doctrine\ODM\MongoDB\Mapping\Annotations\Field;
use Doctrine\ODM\MongoDB\Mapping\Annotations\Id;
use Symfony\Component\Validator\Constraints\NotBlank;

class Document
{
/**
* @Id()
*/
private ?string $id = null;

/**
* @NotBlank(allowNull=true)
* @Field(type="string")
*/
private ?string $field = null;

public function getId(): ?string
{
return $this->id;
}

public function setField(
?string $field
): self {
$this->field = $field;
}

public function getField(): ?string
{
return $this->field;
}
}
```

API Platform will 400 when setting `field` to `null` even though it is nullable (as far as mongodb and PHP are concerned).

`The type of the "field" attribute must be "string", "NULL" given.`

```sh
curl -i -H 'Accept: application/json' -H 'Content-Type: application/merge-patch+json' -X PATCH -d '{"field": null}' http://***/api/items/5f7d021b0cf6b87a7f6d8a12; echo
HTTP/1.1 400 Bad Request
Server: nginx/1.19.2
Content-Type: application/problem+json; charset=utf-8
Transfer-Encoding: chunked
Connection: keep-alive
X-Powered-By: PHP/7.4.10
X-Content-Type-Options: nosniff
X-Frame-Options: deny
Cache-Control: no-cache, private
Date: Wed, 07 Oct 2020 03:40:10 GMT
X-Debug-Token: 2b9265
X-Debug-Token-Link: http://***/_profiler/2b9265
X-Robots-Tag: noindex
Link: ; rel="http://www.w3.org/ns/hydra/core#apiDocumentation"
X-Previous-Debug-Token: cf061e

{"type":"https:\/\/tools.ietf.org\/html\/rfc2616#section-10","title":"An error occurred","detail":"The type of the \u0022field\u0022 attribute must be \u0022string\u0022, \u0022NULL\u0022 given."...}
```

**Possible Solution**

`ApiPlatform\Core\Bridge\Doctrine\MongoDbOdm\PropertyInfo\DoctrineExtractor` shouldn't read nullable from `Doctrine\ODM\MongoDB\Mapping\ClassMetadata` to determine if the `Symfony\Component\PropertyInfo\Type` is nullable. In mongodb it's always nullable. The closest thing to notnullable is adding a `Symfony\Component\Validator\Constraints\NotBlank` constraint.

**Note: This patch has in no way been tested, just an example of what I mean.**

```patch
diff --git a/src/Bridge/Doctrine/MongoDbOdm/PropertyInfo/DoctrineExtractor.php b/src/Bridge/Doctrine/MongoDbOdm/PropertyInfo/DoctrineExtractor.php
index 3325392b..22d1cf2b 100644
--- a/src/Bridge/Doctrine/MongoDbOdm/PropertyInfo/DoctrineExtractor.php
+++ b/src/Bridge/Doctrine/MongoDbOdm/PropertyInfo/DoctrineExtractor.php
@@ -66,9 +66,7 @@ final class DoctrineExtractor implements PropertyListExtractorInterface, Propert
$class = $metadata->getAssociationTargetClass($property);

if ($metadata->isSingleValuedAssociation($property)) {
- $nullable = $metadata instanceof MongoDbClassMetadata && $metadata->isNullable($property);
-
- return [new Type(Type::BUILTIN_TYPE_OBJECT, $nullable, $class)];
+ return [new Type(Type::BUILTIN_TYPE_OBJECT, true, $class)];
}

$collectionKeyType = Type::BUILTIN_TYPE_INT;
@@ -87,19 +85,17 @@ final class DoctrineExtractor implements PropertyListExtractorInterface, Propert

if ($metadata->hasField($property)) {
$typeOfField = $metadata->getTypeOfField($property);
- $nullable = $metadata instanceof MongoDbClassMetadata && $metadata->isNullable($property);
-
switch ($typeOfField) {
case MongoDbType::DATE:
- return [new Type(Type::BUILTIN_TYPE_OBJECT, $nullable, 'DateTime')];
+ return [new Type(Type::BUILTIN_TYPE_OBJECT, true, 'DateTime')];
case MongoDbType::HASH:
- return [new Type(Type::BUILTIN_TYPE_ARRAY, $nullable, null, true)];
+ return [new Type(Type::BUILTIN_TYPE_ARRAY, true, null, true)];
case MongoDbType::COLLECTION:
- return [new Type(Type::BUILTIN_TYPE_ARRAY, $nullable, null, true, new Type(Type::BUILTIN_TYPE_INT))];
+ return [new Type(Type::BUILTIN_TYPE_ARRAY, true, null, true, new Type(Type::BUILTIN_TYPE_INT))];
default:
$builtinType = $this->getPhpType($typeOfField);

- return $builtinType ? [new Type($builtinType, $nullable)] : null;
+ return $builtinType ? [new Type($builtinType, true)] : null;
}
}
```