samples: mesh_badge: fix unknown sensor status message

When the value of an unknown sensor property ID is requested, the length
field should represent the value of zero, as explain in the comment in
sens_unknown_fill. However the marshalled representation of a zero
length is wrong in the code. The format A uses a 1-based uint4 length
value, so the range 0x0–0xF represents the range 1-16. The zero length
is represented by format B using the 0x7F value.

Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>