T6138: Fix op-mode show conntrack table with flowtable offloads

The op-mode command show conntrack table ipv4 fails if gets a
conntrack entrie with flowtable offload. Those entries do not
have key timeout

File "/usr/libexec/vyos/op_mode/conntrack.py", line 115, in get_formatted_output
  timeout = meta['timeout']
            ~~~~^^^^^^^^^^^

Use the timeout n/a for those offload conntrack entries