Fix meta update in gen exchange (for meta service string updates)

- was assuming all meta changes were related to bit sets
- was returning fail, when successful

git-svn-id: http://svn.code.sf.net/p/retroshare/code/trunk@7397 b45a01b8-16f6-495d-af2f-9b41ad6348cc

libretroshare/src/util/contentvalue.cc

@@ -294,6 +294,10 @@ void ContentValue::clear(){
     clearData();
 }
 
+bool ContentValue::empty() const{
+    return mKvSet.empty();
+}
+
 void ContentValue::clearData(){
 
     std::map<std::string, std::pair<uint32_t, char*> >::iterator