another null check

Author: rithikanarayan

tracer/src/Datadog.Trace/ClrProfiler/AutoInstrumentation/AWS/Lambda/LambdaCommon.cs

@@ -128,9 +128,12 @@ private static void WriteRequestPayload(WebRequest request, string data)
 
     private static void WriteRequestHeaders(WebRequest request, IDictionary<string, string> context)
     {
-        foreach (var kv in context)
+        if (context != null)
         {
-            request.Headers.Add(kv.Key, kv.Value);
+            foreach (var kv in context)
+            {
+                request.Headers.Add(kv.Key, kv.Value);
+            }
         }
     }