Fix potential NullReferenceException and added comment why cannot use .Values directly

Author: ramonsmits

src/ServiceControl.Audit.Persistence.RavenDB/Indexes/MessagesViewIndex.cs

@@ -27,7 +27,7 @@ from message in messages
                     DeliveryTime = (TimeSpan?)metadata["DeliveryTime"],
                     Query = new[] {
                         string.Join(' ', message.Headers.Values),
-                        string.Join(' ', metadata.Values.Select(v => v.ToString() ?? string.Empty))
+                        string.Join(' ', metadata.Values.Where(v => v != null).Select(v => v.ToString())) // Needed, RaveDB does not like object arrays
                     },
                     ConversationId = (string)metadata["ConversationId"]
                 };

src/ServiceControl.Persistence.RavenDB/Indexes/MessagesViewIndex.cs

@@ -33,7 +33,10 @@ from message in messages
                     CriticalTime = (TimeSpan?)metadata["CriticalTime"],
                     ProcessingTime = (TimeSpan?)metadata["ProcessingTime"],
                     DeliveryTime = (TimeSpan?)metadata["DeliveryTime"],
-                    Query = metadata.Select(_ => _.Value.ToString()).Union(new[] { string.Join(" ", last.Headers.Select(x => x.Value)) }).ToArray(),
+                    Query = new[] {
+                        string.Join(' ', last.Headers.Values),
+                        string.Join(' ', metadata.Values.Where(v => v != null).Select(v => v.ToString())) // Needed, RavenDB does not like object arrays
+                    },
                     ConversationId = (string)metadata["ConversationId"]
                 };