Project.pm: allow unset config values to remain undef
When loading values from the local project's config file, any values
that do not have a value still end up getting loaded and assigned
to the object with a value of "" (or possibly a different default
value), but they always create the key in the project object's hash.
Change this so that if the default value is specified explicitly
as "undef" then when the config value does not exist no key will
be created in the project object's hash for it at all.
Signed-off-by: Kyle J. McKay <mackyle@gmail.com>