Only add thread name if not null

Test to only add the porperty iff Thread.Name is not null.

Author: StefanOssendorf

src/Serilog.Enrichers.Thread/Enrichers/ThreadNameEnricher.cs

@@ -38,7 +38,11 @@ public class ThreadNameEnricher : ILogEventEnricher
         /// <param name="propertyFactory">Factory for creating new properties to add to the event.</param>
         public void Enrich(LogEvent logEvent, ILogEventPropertyFactory propertyFactory) 
         {
-            logEvent.AddPropertyIfAbsent(new LogEventProperty(ThreadNamePropertyName, new ScalarValue(Thread.CurrentThread.Name)));
+            var threadName = Thread.CurrentThread.Name;
+            if (threadName != null) 
+            {
+                logEvent.AddPropertyIfAbsent(new LogEventProperty(ThreadNamePropertyName, new ScalarValue(Thread.CurrentThread.Name)));
+            }
         }
     }
 #endif