Improve null-safety of module/spring-boot-webflux

See gh-46926

Author: mhalbritter

module/spring-boot-webflux/src/main/java/org/springframework/boot/webflux/actuate/endpoint/web/AbstractWebFluxEndpointHandlerMapping.java

@@ -433,8 +433,8 @@ private Mono<ResponseEntity<Object>> handleResult(@Nullable Publisher<?> result,
 			if (result instanceof Flux) {
 				result = ((Flux<?>) result).collectList();
 			}
-			return Mono.from(result)
-				.map(this::toResponseEntity)
+			Mono<?> mono = (result != null) ? Mono.from(result) : Mono.empty();
+			return mono.map(this::toResponseEntity)
 				.onErrorMap(InvalidEndpointRequestException.class,
 						(ex) -> new ResponseStatusException(HttpStatus.BAD_REQUEST, ex.getReason()))
 				.defaultIfEmpty(new ResponseEntity<>(

module/spring-boot-webflux/src/main/java/org/springframework/boot/webflux/autoconfigure/error/DefaultErrorWebExceptionHandler.java

@@ -130,10 +130,15 @@ protected Mono<ServerResponse> renderErrorView(ServerRequest request) {
 		return Flux.just(getData(status).toArray(new String[] {}))
 			.flatMap((viewName) -> renderErrorView(viewName, responseBody, errorAttributes))
 			.switchIfEmpty(this.errorProperties.getWhitelabel().isEnabled()
-					? renderDefaultErrorView(responseBody, errorAttributes) : Mono.error(getError(request)))
+					? renderDefaultErrorView(responseBody, errorAttributes) : getErrorMono(request))
 			.next();
 	}
 
+	private Mono<ServerResponse> getErrorMono(ServerRequest request) {
+		Throwable error = getError(request);
+		return (error != null) ? Mono.error(error) : Mono.empty();
+	}
+
 	private List<String> getData(int errorStatus) {
 		List<String> data = new ArrayList<>();
 		data.add("error/" + errorStatus);