Improved null_logger factory

include/spdlog/sinks/null_sink.h

@@ -33,13 +33,17 @@ using null_sink_st = null_sink<details::null_mutex>;
 template<typename Factory = default_factory>
 inline std::shared_ptr<logger> null_logger_mt(const std::string &logger_name)
 {
-    return Factory::template create<sinks::null_sink_mt>(logger_name);
+    auto null_logger = Factory::template create<sinks::null_sink_mt>(logger_name);
+    null_logger->set_level(level::off);
+    return null_logger;
 }
 
 template<typename Factory = default_factory>
 inline std::shared_ptr<logger> null_logger_st(const std::string &logger_name)
 {
-    return Factory::template create<sinks::null_sink_st>(logger_name);
+    auto null_logger = Factory::template create<sinks::null_sink_st>(logger_name);
+    null_logger->set_level(level::off);
+    return null_logger;
 }
 
 } // namespace spdlog