Idiomatic way of specifying mixed return type

[MAILPOET-4191]

mailpoet/lib/Automation/Engine/Workflows/Field.php

@@ -54,7 +54,9 @@ class Field {
     return $this->factory;
   }
 
-  // @phpstan-ignore-next-line - there is no consistent return type
+  /**
+   * @return mixed
+   */
   public function getValue() {
     return $this->getFactory()();
   }