Fact table foreign keys null?

You can leave the FK to some dimension tables as NULL if those dimensions are not known or not applicable. You just have to remember to use outer joins when you do your reporting query.

Alternatively, some people create a "none" and/or an "n/a" dimension record for data mart dimensions and then populate fact table FKs to point at these rather than using NULLs. People who do this like this approach because they have an aversion to outer joins.

People who use NULL FKs in fact tables usually have an aversion to people who have an version to outer joins. ;) (in other words, this is a stylistic issue which can generate religious wars)

I say do whichever you prefer, but pick one approach and stick to it fervently.

Don't put nulls in the Warehouse or in the Marts.

The warehouse should be well normalized (at least BCNF) and therefore should exclude nulls. Nulls might be preserved in staging tables if they exist in data sources but they shouldn't be needed in the warehouse itself.

Marts should be designed to support presentation tools and user queries. Nulls just get in the way of those things because they are never displayed and they make user queries more complex and error-prone - especially in foreign key columns which are frequently subject to joins.