code: public final Mono<T> defaultIfEmpty(T defaultV) {
  	    if (this instanceof Fuseable.ScalarCallable) {
  		    try {
  			    T v = block();
  			    if (v == null) {
  				    return Mono.just(defaultV);
  			    }
  		    }
  		    catch (Throwable e) {
  			    //leave MonoError returns as this
  		    }
  		    return this;
  	    }
  		return onAssembly(new MonoDefaultIfEmpty<>(this, defaultV));
  	}
docs: Provide a default single value if this mono is completed without any data
  
  <p>
  <img class="marble" src="doc-files/marbles/defaultIfEmpty.svg" alt="">
  <p>
  @param defaultV the alternate value if this sequence is empty
  
  @return a new {@link Mono}
  
  @see Flux#defaultIfEmpty(Object)