OPTIM: log: declare empty buffer as global variable

OPTIM: log: declare empty buffer as global variable

'empty' buffer used in sess_build_logline() inside a loop, and since it
is only being read from and not modified, until recently it ended up being
cached most of the time and didn't cause overhead due to systematic push
on the stack.

However, due recent encoding work and new added variables on the stack,
we're starting to reach a stack limit and declaring 'empty' buffer within
the loop seems to cause non-negligible CPU overhead.

Since the variable isn't modified during log generation, let's declare
'empty' buffer as a global variable outside from sess_build_logline()
to prevent pushing it on the stack for each node evaluation.