pfcwd show stats crash with invalid queue name

[neethajohn]

Description
'pfcwd show stats' crashes when an invalid queue name is provided as its argument

Steps to reproduce the issue
Issue the command "pfcwd show stats Ethernet400:0". Ethernet400:0 should be an invalid queue name.

Describe the results you received
admin@str-7260cx3-acs-7:~$ pfcwd show stats Ethernet400:0
Traceback (most recent call last):
File "/usr/bin/pfcwd", line 12, in
sys.exit(cli())
File "/usr/lib/python2.7/dist-packages/click/core.py", line 722, in call
return self.main(*args, **kwargs)
File "/usr/lib/python2.7/dist-packages/click/core.py", line 697, in main
rv = self.invoke(ctx)
File "/usr/lib/python2.7/dist-packages/click/core.py", line 1066, in invoke
return _process_result(sub_ctx.command.invoke(sub_ctx))
File "/usr/lib/python2.7/dist-packages/click/core.py", line 1066, in invoke
return _process_result(sub_ctx.command.invoke(sub_ctx))
File "/usr/lib/python2.7/dist-packages/click/core.py", line 895, in invoke
return ctx.invoke(self.callback, **ctx.params)
File "/usr/lib/python2.7/dist-packages/click/core.py", line 535, in invoke
return callback(*args, **kwargs)
File "/usr/lib/python2.7/dist-packages/pfcwd/main.py", line 87, in stats
stats = db.get_all(db.COUNTERS_DB, 'COUNTERS:' + queue_oid)
TypeError: cannot concatenate 'str' and 'NoneType' objects

Describe the results you expected

admin@str-7260cx3-acs-7:~$ pfcwd show stats Ethernet400:0                        
  QUEUE    STATUS    STORM DETECTED/RESTORED    TX OK/DROP    RX OK/DROP    TX LAST OK/DROP    RX LAST OK/DROP
-------  --------  -------------------------  ------------  ------------  -----------------  -----------------

Additional information you deem important (e.g. issue happens only occasionally)

Output of show version

SONiC Software Version: SONiC.20191130.45
Distribution: Debian 9.13
Kernel: 4.9.0-11-2-amd64
Build commit: 56b9bd90e
Build date: Mon Aug 17 08:48:04 UTC 2020
Built by: sonicbld@jenkins-slave-phx-2

Platform: x86_64-arista_7260cx3_64
HwSKU: Arista-7260CX3-Q64
ASIC: broadcom
Serial Number: SSJ17450492
Uptime: 23:34:35 up 1 day, 4 min,  2 users,  load average: 1.58, 1.60, 1.64

Docker images:
REPOSITORY                 TAG                 IMAGE ID            SIZE
docker-syncd-brcm-rpc      latest              27920becb5f7        682MB
docker-syncd-brcm          latest              27920becb5f7        682MB
docker-fpm-frr             20191130.45         34b39fe944d6        333MB
docker-fpm-frr             latest              34b39fe944d6        333MB
docker-snmp-sv2            20191130.45         468d99f4713a        347MB
docker-snmp-sv2            latest              468d99f4713a        347MB
docker-acms                20191130.45         49d6ce54c7de        181MB
docker-acms                latest              49d6ce54c7de        181MB
docker-lldp-sv2            20191130.45         425500aa0d18        310MB
docker-lldp-sv2            latest              425500aa0d18        310MB
docker-orchagent           20191130.45         c2651bdd5e90        332MB
docker-orchagent           latest              c2651bdd5e90        332MB
docker-teamd               20191130.45         8bba13bf4e37        313MB
docker-teamd               latest              8bba13bf4e37        313MB
docker-syncd-brcm          20191130.45         78d678c4abb8        436MB
docker-platform-monitor    20191130.45         6ee0ba43b69e        356MB
docker-platform-monitor    latest              6ee0ba43b69e        356MB
docker-sonic-telemetry     20191130.45         cccc9da859b3        353MB
docker-sonic-telemetry     latest              cccc9da859b3        353MB
docker-database            20191130.45         71a0b9cf9ea0        289MB
docker-database            latest              71a0b9cf9ea0        289MB
docker-router-advertiser   20191130.45         45959a6bab33        289MB
docker-router-advertiser   latest              45959a6bab33        289MB
docker-dhcp-relay          20191130.45         be2c2f1b81ce        299MB
docker-dhcp-relay          latest              be2c2f1b81ce        299MB
k8s.gcr.io/pause           3.2                 80d28bedfe5d        683kB