Fix PROCESS_FIELDS to handle empty arguments (alibaba#1128)

Fix PROCESS_FIELDS to handle empty arguments

Author: NewbieOrange

rpc/serialize.h

@@ -278,10 +278,10 @@ namespace rpc
         ValueType m_checksum;
     };
 
-#define PROCESS_FIELDS(...)                     \
-        template<typename AR>                   \
-        void process_fields(AR& ar) {           \
-            return reduce(ar, __VA_ARGS__);     \
+#define PROCESS_FIELDS(...)                        \
+        template<typename AR>                      \
+        void process_fields(AR& ar) {              \
+            return reduce(ar, ## __VA_ARGS__);     \
         }
 
     template<typename K, typename V>