Fix serialization of empty sub-nvlists

[rstone]

sys/kern/subr_nvlist.c

367

If the sub-nvlist is empty, then nvp will be NULL here. Running "continue;" here will cause the while loop to terminate prematurely without finishing the serialization of our parent. Fixing this with a goto is quite ugly but I couldn't see a very good way to get around it

563

oops. Will fix unnecessary braces

[rstone]

Update unit test to test serializing an empty nvlist

[rstone]

Already fixed in r277925