defaultIfEmpty method
Emits a given value if this Observable completes without emitting any value, otherwise mirrors the source.
Implementation
Observable<T> defaultIfEmpty(T value) =>
    DefaultIfEmptyObservable<T>(this, value);Emits a given value if this Observable completes without emitting any value, otherwise mirrors the source.
Observable<T> defaultIfEmpty(T value) =>
    DefaultIfEmptyObservable<T>(this, value);