To add a null value, use null() or an empty string ''
Do you load the entire application? It depends how long the load takes. If the whole app loads in a few minutes then I wouldn't bother with an incremental load as the QlikView server reloads the app in the background.
Search for Incremental Load for more details.
You can modify the error messages in charts - edit the chart Properties / General / Error Messages
While developing the application, daily do we need to reload the application? Because the data is huge, already we are creating set of QVDs through incremental load concept and using those QVDs as input. But while developing the application itself, how can proceed with loads?