Handle empty datafolder better.

If datafolder is erased, the default value will be shown as a placeholder.

If installation is submitted without a datafolder the default value will be used.
Hint to default data directory

not to before submitted data directory.

Thanks to bartv2 for the heads up.
Merge pull request #2915 from Kondou-ger/handle_empty_datafolder_better

Handle empty datafolder better.