Added is_empty() method to transaction class

Method is_empty() can be determined if there have no been changes
to the transaction.

include/wsrep/sr_key_set.hpp

@@ -51,6 +51,7 @@ namespace wsrep
 
         const branch_type& root() const { return root_; }
         void clear() { root_.clear(); }
+        bool empty() const { return root_.empty(); }
     private:
         branch_type root_;
     };

include/wsrep/transaction.hpp

@@ -98,6 +98,14 @@ namespace wsrep
             return (streaming_context_.fragments_certified() > 0);
         }
 
+        /**
+         * Return true if transaction has not generated any changes.
+         */
+        bool is_empty() const
+        {
+            return sr_keys_.empty();
+        }
+
         bool pa_unsafe() const { return pa_unsafe_; }
         void pa_unsafe(bool pa_unsafe) { pa_unsafe_ = pa_unsafe; }