Add default value for number of PSUs

[spilkey-cisco]

- What I did

Fixed possible snmp error if PSU data is not populated in state DB by pmon by the time snmp tries to retrieve the data.

- How I did it

Wrap the int conversion with a try-except block, and return 0 if the database did not contain a number in the psu_num field.

- How to verify it

Ensure exceptions like the following are not seen in syslog:

 #snmp-subagent [sonic_ax_impl] ERROR: PowerStatusHandler._get_psu_index() caught an unexpected exception during _get_num_psus()
 #012Traceback (most recent call last):
 #012  File "/usr/local/lib/python3.9/dist-packages/sonic_ax_impl/mibs/vendor/cisco/ciscoEntityFruControlMIB.py", line 101, in _get_psu_index
 #012    num_psus = self._get_num_psus()
 #012  File "/usr/local/lib/python3.9/dist-packages/sonic_ax_impl/mibs/vendor/cisco/ciscoEntityFruControlMIB.py", line 66, in _get_num_psus
 #012    return int(num_psus[0])
 #012ValueError: invalid literal for int() with base 10: ''

- Description for the changelog

Add default value for number of PSUs

[mssonicbld]

/azp run

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