Resolve #3491027 "Php fatal error"

Closes #3491027

src/Plugin/Validation/ParallelWorkspaceConflictConstraintValidator.php

@@ -7,6 +7,6 @@ use Symfony\Component\Validator\Constraint;
 
 class ParallelWorkspaceConflictConstraintValidator extends EntityWorkspaceConflictConstraintValidator {
 
-  public function validate($entity, Constraint $constraint) {}
+  public function validate($entity, Constraint $constraint): void {}
 
 }