ContentCatalog#addFile should not compute out if out property is set but falsy
Currently, the ContentCatalog#addFile always computes the out if the out property is set but falsy. This turns a non-publishable page into a publishable page, and can also cause the method to crash (because it requires many properties to be set). If the out property is set (i.e., the key is present), but the value is falsy, the out property should be removed rather than computed and assigned.
(As an alternative, we could look for a "publishable" property in the input to determine whether or not to compute the out).
This capability is important when importing virtual files from other sources that should not be published.
Edited by Dan Allen