"" looks better than "?" (part of #722)

campcaster/src/products/gLiveSupport/src/GLiveSupport.cxx

@@ -1855,7 +1855,7 @@ GLiveSupport :: substituteRdsItem(Ptr<Glib::ustring>::Ref   rdsString,
             value = playable->getMetadata(metadataKey);
         }
         if (!value) {
-            value.reset(new Glib::ustring("?"));
+            value.reset(new Glib::ustring(""));
         }
         rdsString->replace(pos, placeholder.length(), *value);
     }

campcaster/src/products/gLiveSupport/src/GLiveSupport.h

@@ -381,7 +381,7 @@ class GLiveSupport : public LocalizedConfigurable,
 
         /**
          *  Replace a single placeholders in the RDS settings.
-         *  If the corresponding metadata is not found, a "?" character
+         *  If the corresponding metadata is not found, an empty string
          *  is substituted instead.
          *
          *  @param  rdsString   the string with the placeholders;