framework: define static empty string for return

framework/cybertron/record/record_base.cpp

@@ -58,7 +58,7 @@ const std::string& RecordBase::GetMessageType(
   if (search != channel_message_type_map_.end()) {
     return search->second;
   }
-  return "";
+  return g_empty_string;
 }
 
 const std::string& RecordBase::GetProtoDesc(
@@ -67,7 +67,7 @@ const std::string& RecordBase::GetProtoDesc(
   if (search != channel_proto_desc_map_.end()) {
     return search->second;
   }
-  return "";
+  return g_empty_string;
 }
 
 }  // namespace record

framework/cybertron/record/record_base.h

@@ -33,6 +33,8 @@ namespace apollo {
 namespace cybertron {
 namespace record {
 
+static std::string g_empty_string = "";
+
 class RecordBase {
  public:
   RecordBase();