[ieee802_1ab]Ensuring that the LLDP_system_data is non-empty before getting field from it.

[sdddean]

Signed-off-by: sundandan sundandan@asterfusion.com

What I did
Add a non-empty protection before getting field form local_chassis_date.

Why I did it
As the logic in sonic-dbsyncd, there will be no 'LLDP_LOC_CHASSIS' table in APP DB until we establish neighbor relationships with LLDP neighbors.
When there is no 'LLDP_LOC_CHASSIS' table in APP DB, walk node .1.0.8802.1.1.2.1.3 will led to error log:

Jun 4 03:30:18.191672 leaf-229 ERR snmp#snmp-subagent [ax_interface] ERROR: MIBUpdater.start() caught an unexpected exception during update_data()#012Traceback (most recent call last):#12 File "/usr/local/lib/python3.6/dist-packages/ax_interface/mib.py", line 37, in start#012 self.reinit_data()#12 File "/usr/local/lib/python3.6/dist-packages/sonic_ax_impl/mibs/ieee802_1ab.py", line 115, in reinit_data#012 self.loc_chassis_data[b'lldp_loc_sys_cap_supported'] = parse_sys_capability(self.loc_chassis_data[b'lldp_loc_sys_cap_supported'])#012TypeError: 'NoneType' object is not subscriptable

How I verified it
The error log described above does not appear anymore.