What is Java empty constructor?

There is no benefit unless you are interacting with a tool or framework that detects your class's declared methods and constructors via reflection and then silently does different things depending on whether it finds the default Java empty constructor or not. (I seem to remember that early versions of Spring or Hibernate did this.)

Otherwise, as you say, any inconsistency would be flagged by the compiler, so there is no point in declaring things that would be auto-generated anyway.