DATAREDIS-779 - Emit false if a conditional ReactiveStringCommands.set(…) does not set a value.

We now emit false when SET XX/SET NX complete without setting the value because the condition was evaluated to not set the value. Previously, we did not emit a value as Redis responds null (nil). This behavior becomes visible through ReactiveValueOperations.set[ifPresent|ifAbsent](…) methods.

This is a breaking change in the sense that changes the character of ReactiveValueOperations.set[ifPresent|ifAbsent](…) from an optionally emitted element to always emit. Reactive sequences that currently resume operation with .switchIfEmpty(…) will no longer utilize the fallback publisher.

Original pull request: #317.

src/main/java/org/springframework/data/redis/connection/lettuce/LettuceReactiveStringCommands.java

@@ -37,6 +37,7 @@ import org.springframework.util.Assert;
 /**
  * @author Christoph Strobl
  * @author Mark Paluch
+ * @author Jiahe Cai
  * @since 2.0
  */
 class LettuceReactiveStringCommands implements ReactiveStringCommands {
@@ -93,7 +94,9 @@ class LettuceReactiveStringCommands implements ReactiveStringCommands {
 
 			Mono<String> mono = args != null ? cmd.set(command.getKey(), command.getValue(), args)
 					: cmd.set(command.getKey(), command.getValue());
-			return mono.map(LettuceConverters::stringToBoolean).map((value) -> new BooleanResponse<>(command, value));
+			return mono.map(LettuceConverters::stringToBoolean)
+					.map(value -> new BooleanResponse<>(command, value))
+					.switchIfEmpty(Mono.just(new BooleanResponse<>(command, Boolean.FALSE)));
 		}));
 	}