Have TimeSeriesProperty return an empty default rather than throwing.

This seems more reasonable as thats what really happens.
Refs #9045

Code/Mantid/Framework/Kernel/src/TimeSeriesProperty.cpp

@@ -1674,7 +1674,7 @@ namespace Mantid
     template<typename TYPE>
     std::string TimeSeriesProperty<TYPE>::getDefault() const
     {
-      throw Exception::NotImplementedError("TimeSeries properties don't have defaults");
+      return ""; // No defaults can be provided=empty string
     }
 
     /**