iot/zoap: Fix handling of 16-bytes block-wise transfers

This fixes the case that a request for block number (NUM) 0, using a
16 byte block was considered invalid.

This was because it is encoded as the value 0 (zero), which can be
expressed as the BLOCK1 option present but without any value
associated. The old code considered this the same as the option not
existing.

Change-Id: I0f3912803a88865e9f544a6d0078ed4231775a88
Signed-off-by: Vinicius Costa Gomes <vinicius.gomes@intel.com>