[pfc] Fix variable return type

[neethajohn]

Signed-off-by: Neetha John [email protected]

Description of PR

Fixes the error seen during test case collection introduced by #2599. Initialize the variable as a list
https://sonic-jenkins.westus2.cloudapp.azure.com/job/common/job/sonic-mgmt-testing/job/sonic-mgmt-pr/2462/consoleFull

____________________________________________________ ERROR collecting pfc/test_pfc_pause_lossless.py ______________________________________________________
/usr/local/lib/python2.7/dist-packages/pluggy/hooks.py:286: in __call__
    return self._hookexec(self, self.get_hookimpls(), kwargs)
/usr/local/lib/python2.7/dist-packages/pluggy/manager.py:93: in _hookexec
    return self._inner_hookexec(hook, methods, kwargs)
/usr/local/lib/python2.7/dist-packages/pluggy/manager.py:87: in <lambda>
    firstresult=hook.spec.opts.get("firstresult") if hook.spec else False,
/usr/local/lib/python2.7/dist-packages/_pytest/python.py:234: in pytest_pycollect_makeitem
    res = list(collector._genfunctions(name, obj))
/usr/local/lib/python2.7/dist-packages/_pytest/python.py:410: in _genfunctions
    self.ihook.pytest_generate_tests(metafunc=metafunc)
/usr/local/lib/python2.7/dist-packages/pluggy/hooks.py:286: in __call__
    return self._hookexec(self, self.get_hookimpls(), kwargs)
/usr/local/lib/python2.7/dist-packages/pluggy/manager.py:93: in _hookexec
    return self._inner_hookexec(hook, methods, kwargs)
/usr/local/lib/python2.7/dist-packages/pluggy/manager.py:87: in <lambda>
    firstresult=hook.spec.opts.get("firstresult") if hook.spec else False,
conftest.py:748: in pytest_generate_tests
    metafunc.parametrize("enum_dut_lossless_prio", generate_priority_lists(metafunc, 'lossless'))
/usr/local/lib/python2.7/dist-packages/_pytest/python.py:1004: in parametrize
    function_definition=self.definition,
/usr/local/lib/python2.7/dist-packages/_pytest/mark/structures.py:125: in _for_parametrize
    parameters = cls._parse_parametrize_parameters(argvalues, force_tuple)
/usr/local/lib/python2.7/dist-packages/_pytest/mark/structures.py:119: in _parse_parametrize_parameters
    ParameterSet.extract_from(x, force_tuple=force_tuple) for x in argvalues
E   TypeError: 'NoneType' object is not iterable

Type of change

• Bug fix
• Testbed and Framework(new/improvement)
• Test case(new/improvement)

How did you verify/test it?

Tried test case collection with the fix. No longer seeing the error