Silently pass {{undefined}} as empty strings

This corrects to the intended behavior of an earlier patch which
intended to create this functionality.

Undefined flags such as {{build_with}} no longer need to be specified in
profiles.  It is my intention that we will eventually add package-level
defaults, since implicit behavior can be dangerous to users.
Merge pull request #195 from hashdist/default_subs_empty

Silently pass {{undefined}} as empty strings