Don't choke on nil replies from Redis.

src/backends/redis/batch.cpp

@@ -64,6 +64,8 @@ namespace redis {
 				return ErrorString(parReply->str, parReply->len);
 			case REDIS_REPLY_STATUS:
 				return StatusString(parReply->str, parReply->len);
+			case REDIS_REPLY_NIL:
+				return nullptr;
 			default:
 				assert(false); //not reached
 				return Reply();

src/backends/redis/reply.cpp

@@ -26,6 +26,10 @@ namespace redis {
 	}
 
 	const std::string& get_string (const Reply& parReply) {
+		static const std::string empty_str;
+		if (RedisVariantType_Nil == parReply.which())
+			return empty_str;
+
 		assert(RedisVariantType_String == parReply.which());
 		return boost::get<std::string>(parReply);
 	}

src/backends/redis/reply.hpp

@@ -53,7 +53,8 @@ namespace redis {
 			std::string,
 			std::vector<Reply>,
 			ErrorString,
-			StatusString
+			StatusString,
+			std::nullptr_t
 		>;
 	} //namespace implem
 	enum RedisVariantTypes {
@@ -61,7 +62,8 @@ namespace redis {
 		RedisVariantType_String,
 		RedisVariantType_Array,
 		RedisVariantType_Error,
-		RedisVariantType_Status
+		RedisVariantType_Status,
+		RedisVariantType_Nil
 	};
 
 	struct Reply : implem::RedisVariantType {
@@ -73,6 +75,7 @@ namespace redis {
 		Reply ( std::vector<Reply>&& parVal ) : base_class(std::move(parVal)) {}
 		Reply ( ErrorString&& parVal ) : base_class(std::move(parVal)) {}
 		Reply ( StatusString&& parVal ) : base_class(std::move(parVal)) {}
+		Reply ( std::nullptr_t parVal ) : base_class(parVal) {}
 		~Reply ( void ) noexcept = default;
 	};