That's a bad assumption to make, even though right now it holds
(because of the way we've implemented decompression of packfiles),
this may change in the future, given that ODB objects can be
binary data.

Furthermore, the ODB object can return a NULL pointer if the object
is empty. Copying the NULL pointer to the strbuf lets us handle it
like an empty string. Again, the NULL pointer is valid behavior because
you're supposed to check the *size* of the object before working
on it.