[aclshow] fix aclshow when clear is called before counters are populated

[stepanblyschak]

Signed-off-by: Stepan Blyschak stepanb@nvidia.com

What I did

Fixed a bug that in case ACL counters are cleared before counters are populated in COUNTERS DB the next aclshow will fail:

admin@sonic:~$ aclshow -c -t DATAACL
admin@sonic:~$ aclshow -a -t DATAACL
'NoneType' object is not subscriptable  

How I did it

Keep self.acl_counters in sync with COUNTERS DB and don't put empty values when no counters are available, so when dumping counters and loading them back there are no empty values.

How to verify it

Without this change the added UT fails:

self = <aclshow.AclStat object at 0x7f0208277438>, key = ('DATAACL_NO_COUNTER', 'RULE_NO_COUNTER'), type = 'SAI_ACL_COUNTER_ATTR_PACKETS'
                                                                                     
    def get_counter_value(self, key, type):                                          
        """                                                                          
        return the counter value or the difference comparing with the saved value in string format
        """                                                                          
        if not self.acl_counters[key]:                                               
            return 'N/A'                                                             
                                                                                     
        if key in self.saved_acl_counters: 
>           new_value = int(self.acl_counters[key][type]) - int(self.saved_acl_counters[key][type])
E           TypeError: 'NoneType' object is not subscriptable  

Previous command output (if the output of a command-line utility has changed)

New command output (if the output of a command-line utility has changed)

NOTES:

Request to cherry-pick to 202111.

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