Set value only if we don't already have one

SomeClass.dart

class SomeClass {
  String foobar;
  
  void someMethod(String newValue) {
    if (foobar == null) {
      foobar = newValue;
    }
  }
  
  // OR //
  
  void someMethod(String newValue) {
    foobar ??= newValue;
  }
  
  // OR //
  
  void someMethod([String optionalNewValue]) {
    foobar ??= (optionalNewValue??"defaultValue");
  }
}