MINOR: stats: protect against future stats fields omissions

MINOR: stats: protect against future stats fields omissions

As seen in commits 33a4461fa ("BUG/MINOR: stats: Fix Lua's `get_stats`
function") and a46b142e8 ("BUG/MINOR: Missing stat_field_names (since
f21d17bb)") it seems frequent to omit to update stats_fields[] when
adding a new ST_F_xxx entry. This breaks Lua's get_stats() and shows
a "(null)" in the header of "show stat", but that one is not detectable
to the naked eye anymore.

Let's add a reminder above the enum declaration about this, and a small
reg tests checking for the absence of "(null)". It was verified to fail
before the last patch above.