add sum aggregation function

s = aggr_table()

s[0] = sum(1)
s[0] = sum(2)
s[0] = sum(3)

print(s[0]) <--- 6

Signed-off-by: zhangwei(Jovi) <jovi.zhangwei@gmail.com>
set n field value of nil as 0

nvalue(nil) is 0 after this change.

This issue was found when use aggr_table in traceio.kp

Signed-off-by: zhangwei(Jovi) <jovi.zhangwei@gmail.com>
use aggregation table in scripts/traceio.kp

Signed-off-by: zhangwei(Jovi) <jovi.zhangwei@gmail.com>