Update LoggingEventSource.cs

src/libraries/Microsoft.Extensions.Logging.EventSource/src/LoggingEventSource.cs

@@ -329,7 +329,7 @@ private void FireChangeToken()
         [NonEvent]
         private static LoggerFilterRule[] ParseFilterSpec(string filterSpec, LogLevel defaultLevel)
         {
-            if (filterSpec.Length == 0)
+            if (filterSpec?.Length == 0)
             {
                 return new[] { new LoggerFilterRule(typeof(EventSourceLoggerProvider).FullName, null, defaultLevel, null) };
             }