Fixup incorrect @NotNull annotations

Change-Id: Ib7d5cc09aa1f474617c30f7ac7b0ec43bfe6c00d
Reviewed-on: http://review.couchbase.org/27910
Reviewed-by: Sergey Avseyev <sergey.avseyev@gmail.com>
Reviewed-by: Mark Nunberg <mnunberg@haskalah.org>
Tested-by: Trond Norbye <trond.norbye@gmail.com>