various fixes for null parentId

Author: Ashish Aggarwal

core/src/main/java/com/expedia/www/haystack/client/propagation/TextMapCodex.java

@@ -23,11 +23,11 @@ public class TextMapCodex implements Codex<String,Object> {
 
     @Override
     public String encode(Object value) {
-        return value.toString();
+        return value == null ? null : value.toString();
     }
 
     @Override
     public String decode(Object value) {
-        return value.toString();
+        return  value == null ? null : value.toString();
     }
 }

core/src/main/java/com/expedia/www/haystack/client/propagation/TextMapPropagator.java

@@ -106,7 +106,7 @@ public SpanContext extract(TextMap carrier) {
 
         SpanContext context = new SpanContext(UUID.fromString(traceId),
                                               UUID.fromString(spanId),
-                                              UUID.fromString(parentId));
+                                              parentId == null ? null : UUID.fromString(parentId));
         return context.addBaggage(baggage);
     }