Fix saving of default netclass

This was broken as a side-effect of 3939b31
Default netclass doesn't get the "nets" member
but still needs to be added to the array of classes

Fixes https://gitlab.com/kicad/code/kicad/-/issues/4860

common/project/net_settings.cpp

@@ -76,8 +76,9 @@ NET_SETTINGS::NET_SETTINGS( JSON_SETTINGS* aParent, const std::string& aPath ) :
                         }
 
                         netJson["nets"] = membersJson;
-                        ret.push_back( netJson );
                     }
+
+                    ret.push_back( netJson );
                 }
 
                 return ret;