Add special handling for JSON Strings

Added support to JsonMessageConverter to pass string as is if input type is String
Added guard condition to RSocketListenerFunction to avoid NPE if target function can not be discovered

spring-cloud-function-context/src/main/java/org/springframework/cloud/function/context/config/JsonMessageConverter.java

@@ -17,6 +17,7 @@
 package org.springframework.cloud.function.context.config;
 
 import java.lang.reflect.Type;
+import java.nio.charset.StandardCharsets;
 
 import org.springframework.cloud.function.json.JsonMapper;
 import org.springframework.lang.Nullable;
@@ -76,12 +77,16 @@ public class JsonMessageConverter extends AbstractMessageConverter {
 			return message.getPayload();
 		}
 
+
+
 		Type convertToType = conversionHint == null ? targetClass : (Type) conversionHint;
 		try {
 			return this.jsonMapper.fromJson(message.getPayload(), convertToType);
 		}
 		catch (Exception e) {
-			// ignore
+			if (message.getPayload() instanceof byte[] && targetClass.isAssignableFrom(String.class)) {
+				return new String((byte[]) message.getPayload(), StandardCharsets.UTF_8);
+			}
 		}
 		return null;
 	}