fix: return null for id attribute when targeting key is absent

Per review feedback from dd-oleksii: when targeting key is null,
ResolveAttribute("id") should return null like any other missing
attribute, not throw MissingTargetingKeyException. The shard path
in GetShard() still correctly throws when targeting key is needed.

Author: leoromanovsky

tracer/src/Datadog.Trace/FeatureFlags/FeatureFlagsEvaluator.cs

@@ -417,11 +417,6 @@ internal static int GetShard(string salt, string? targetingKey, int totalShards)
             // Special case "id": if not present, use targeting key
             if (name == "id" && !context.Attributes.ContainsKey(name))
             {
-                if (context.TargetingKey is null)
-                {
-                    throw new MissingTargetingKeyException();
-                }
-
                 return context.TargetingKey;
             }