Return null if deserializing an export timespan doesn't succeed with at least one range.

This should fix an issue with loading old sessions using the old way of
handling session ranges specially.

libs/ardour/export_profile_manager.cc

@@ -437,6 +437,10 @@ ExportProfileManager::deserialize_timespan (XMLNode & root)
 		state->time_format = (TimeFormat) string_2_enum (prop->value(), TimeFormat);
 	}
 
+	if (state->timespans->empty()) {
+		return TimespanStatePtr();
+	}
+
 	return state;
 }