nvl: pack a NULL str as if it were the empty string
nvl: pack a NULL str as if it were the empty string

We shouldn't dereference a NULL struct str when packing it.  Instead we
should treat it as an empty string.  Eventually, we should either make
nvl_set_str() error out when trying to set a NULL, or completely hide the
fact by storing a non-NULL empty str pointer instead.

Signed-off-by: Josef 'Jeff' Sipek <a620f141433c70aa9e43778305a6c68cbfadfb25@josefsipek.net>

jeffpc/libjeffpc,jeffpc/libjeffpc,jeffpc/libjeffpc,jeffpc/libjeffpc