Skip generation of 'empty' copy method

[mkarneim]

As reported in #88 it is confusing if a generated builder provides a copy method that actually does not copy anything:

public ContactBuilder copy(Contact pojo) {
  return self;
}

This happens for example, if the given pojo provides no accessible getter methods (following the Java Beans standard) for its fields:

public class Contact {
  private final String name;
  private final String email;

  @GeneratePojoBuilder(withCopyMethod = true)
  public Contact(String name, String email) {
    this.name = name;
    this.email = email;
  }
}

In such cases PB should skip the generation of the copy method and emit a warning.