Fix more nullable annotations

Author: andrewlock

tracer/src/Datadog.Trace/Activity/DuckTypes/IActivity.cs

@@ -35,9 +35,9 @@ internal interface IActivity : IDuckType
 
         DateTime StartTimeUtc { get; }
 
-        IEnumerable<KeyValuePair<string, string>> Baggage { get; }
+        IEnumerable<KeyValuePair<string, string?>> Baggage { get; }
 
-        IEnumerable<KeyValuePair<string, string>> Tags { get; }
+        IEnumerable<KeyValuePair<string, string?>> Tags { get; }
 
         object AddBaggage(string key, string value);
 

tracer/src/Datadog.Trace/Activity/DuckTypes/IActivity5.cs

@@ -18,7 +18,7 @@ internal interface IActivity5 : IW3CActivity
 
         ActivityKind Kind { get; }
 
-        IEnumerable<KeyValuePair<string, object>> TagObjects { get; }
+        IEnumerable<KeyValuePair<string, object?>> TagObjects { get; }
 
         ActivitySource Source { get; }