A minimum requirement for an installation package is to contain at least a single Feature, which itself contains a single component.
There is however no requirement to add any files to the component.
INSTALLDIR is the root directory of the installation and it requires a value otherwise you will see "Error 2704: Root Directory property undefined" when you attempt to install.
While you don't have to write any files into it, your project must have an INSTALLDIR set. In the General Information view, just set the project's INSTALLDIR to any pre-defined Windows folder shown when you click on the ellipsis. If I was doing this, I think I'd use [TempFolder]. You'll still need at least one Feature and at least one Component (each component's Destination should be [INSTALLDIR]).