createEmptyMapProperty<K, V>  method 
    Null safety
- {String? propertyName}
 
Short hand helper function for initializing an empty EmpireMapProperty.
See EmpireProperty for propertyName usages.
Example
late final EmpireMapProperty planet;
planet = createEmptyMapProperty();
Implementation
EmpireMapProperty<K, V> createEmptyMapProperty<K, V>({String? propertyName}) {
  return EmpireMapProperty(<K, V>{}, this, propertyName: propertyName);
}