add rule hit reason

Author: jianggang

src/main/java/com/featureprobe/sdk/server/model/Toggle.java

@@ -70,7 +70,9 @@ private EvaluationResult hitValue(HitResult hitResult, Object defaultValue, Opti
             } else {
                 res.setValue(variation);
             }
-            res.setReason("Rule " + ruleIndex.get() + " hit");
+            if (ruleIndex.isPresent()) {
+                res.setReason("Rule " + ruleIndex.get() + " hit");
+            }
         }
         return res;
     }