Mark nullable EntityRepository methods as impure

The result of these functions depends on the underlying database state
which can change between multiple invocations. For example:

Calling find() or findOneBy() and asserting the result to be null as a
precondition of a test will generate false errors later in the test when the
underlying record is added and the method is called again

See phpstan#550

Author: nCrazed

stubs/EntityRepository.stub

@@ -20,6 +20,7 @@ class EntityRepository implements ObjectRepository
 	 * @phpstan-param int|null $lockMode
 	 * @phpstan-param int|null $lockVersion
 	 * @phpstan-return TEntityClass|null
+	 * @phpstan-impure
 	 */
 	public function find($id, $lockMode = null, $lockVersion = null);
 
@@ -41,6 +42,7 @@ class EntityRepository implements ObjectRepository
 	 * @phpstan-param array<string, mixed> $criteria The criteria.
 	 * @phpstan-param array<string, string>|null $orderBy
 	 * @phpstan-return TEntityClass|null
+	 * @phpstan-impure
 	 */
 	public function findOneBy(array $criteria, ?array $orderBy = null);