FIX for Issue 4579: PFC config CLI errors out

[vikumarks]

Fix for #4579

What I did

enable_prio = entry.get('pfc_enable').split(',')
update to
enable_prio = entry.get('pfc_enable', '').split(',')

How I did it

entry.get('pfc_enable', '') returns an empty string as the default.
Splitting an empty string gives [''], and the filter on next line (if x.strip()) already strips that out, leaving enable_prio as an empty list

How to verify it

admin@sonic:~$ sudo config interface pfc priority Ethernet24 3 on
Traceback (most recent call last):
  File "/usr/local/bin/pfc", line 8, in <module>
    sys.exit(cli())
             ^^^^^
  File "/usr/local/lib/python3.11/dist-packages/click/core.py", line 764, in __call__
    return self.main(*args, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.11/dist-packages/click/core.py", line 717, in main
    rv = self.invoke(ctx)
         ^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.11/dist-packages/click/core.py", line 1137, in invoke
    return _process_result(sub_ctx.command.invoke(sub_ctx))
                           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.11/dist-packages/click/core.py", line 1137, in invoke
    return _process_result(sub_ctx.command.invoke(sub_ctx))
                           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.11/dist-packages/click/core.py", line 956, in invoke
    return ctx.invoke(self.callback, **ctx.params)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.11/dist-packages/click/core.py", line 555, in invoke
    return callback(*args, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.11/dist-packages/pfc/main.py", line 177, in configPrio
    Pfc(namespace).configPfcPrio(status, interface, priority)
  File "/usr/local/lib/python3.11/dist-packages/utilities_common/multi_asic.py", line 156, in wrapped_run_on_all_asics
    func(self,  *args, **kwargs)
  File "/usr/local/lib/python3.11/dist-packages/pfc/main.py", line 87, in configPfcPrio
    enable_prio = entry.get('pfc_enable').split(',')
                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
AttributeError: 'NoneType' object has no attribute 'split'
admin@sonic:~$ sudo vi /usr/local/lib/python3.11/dist-packages/pfc/main.py
admin@sonic:~$ sudo config interface pfc priority Ethernet24 3 on
admin@sonic:~$

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)

[mssonicbld]

/azp run

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