BUG/MINOR: spoe: Properly detect and skip empty NOTIFY frames

BUG/MINOR: spoe: Properly detect and skip empty NOTIFY frames

Since the SPOE was refactored, the detection of empty NOTIFY frames is
broken. So it is possible to send a NOTIFY frames to an agent with no
message at all. The bug happens because the frame type is now added to the
buffer before the messages encoding. So the buffer is never really empty.

To fix the issue, the condition to detect empty frame was adapted.

This patch must be backported as far as 3.1.