Rolling back to mapping unknown object types as JSON-Schema {"type": "string"}

To avoid BC breaks, we defer this fix to api-platform/core#3403

> **API Platform version(s) affected**: 2.5.x
>
> **Description**
>
> In our tests for `TypeFactory`:
>
> ```
>
>         yield [['type' => 'string'], new Type(Type::BUILTIN_TYPE_OBJECT)];
>         yield [['type' => 'string'], new Type(Type::BUILTIN_TYPE_OBJECT, true)];
>
> // ...
>
>         yield [['type' => 'string'], new Type(Type::BUILTIN_TYPE_OBJECT, false, Dummy::class)];
>         yield [['type' => 'string'], new Type(Type::BUILTIN_TYPE_OBJECT, true, Dummy::class)];
> ```
>
> While reviewing #3402, @dunglas found a potential BC break with objects that may be used in URIs as `string`s (therefore not `objects`):
>
>     * [#3402 (comment)](api-platform/core#3402 (comment))
>
>     * [#3402 (comment)](api-platform/core#3402 (comment))
>
>
> **How to reproduce**
>
> The test should instead convert `object` to `object`:
>
> ```
>
>         yield [['type' => 'object'], new Type(Type::BUILTIN_TYPE_OBJECT)];
>         yield [['type' => 'object'], new Type(Type::BUILTIN_TYPE_OBJECT, true)];
>
> // ...
>
>         yield [['type' => 'object'], new Type(Type::BUILTIN_TYPE_OBJECT, false, Dummy::class)];
>         yield [['type' => 'object'], new Type(Type::BUILTIN_TYPE_OBJECT, true, Dummy::class)];
> ```

Author: Ocramius

TypeFactory.php

@@ -117,7 +117,7 @@ private function makeBasicType(Type $type, string $format = 'json', ?bool $reada
     private function getClassType(?string $className, string $format, ?bool $readableLink, ?array $serializerContext, ?Schema $schema): array
     {
         if (null === $className) {
-            return ['type' => 'object'];
+            return ['type' => 'string'];
         }
 
         if (is_a($className, \DateTimeInterface::class, true)) {
@@ -141,7 +141,7 @@ private function getClassType(?string $className, string $format, ?bool $readabl
 
         // Skip if $schema is null (filters only support basic types)
         if (null === $schema) {
-            return ['type' => 'object'];
+            return ['type' => 'string'];
         }
 
         if ($this->isResourceClass($className) && true !== $readableLink) {