Fixed returning NULL arrays to CIMOM.

Pegasus is especially sensitive about this and reports:

CIM_ERR_FAILED: Traceback (most recent call last):
  File /usr/lib/python2.7/site-packages/cmpi_pywbem_bindings.py, line 82, in __call__
    return self.meth(*args, **kwds)
  File /usr/lib/python2.7/site-packages/cmpi_pywbem_bindings.py, line 514, in get_instance
    cinst = self.pywbem2cmpi_inst(pinst)
  File /usr/lib/python2.7/site-packages/cmpi_pywbem_bindings.py, line 792, in pywbem2cmpi_inst
    cinst.set_property(str(prop.name), data, ctype)
  File /usr/lib/python2.7/site-packages/cmpi.py, line 831, in set_property
    return _cmpi.CMPIInstance_set_property(self, *args)
CIMError: (1, '61')
Add simple provider to test NULL values.

EnumerateInstances on Test_NullArray must succeed without returning CIMError.
Merge pull request #9 from jsafrane/fix-null-array2

Fix returning of NULL arrays from a Python provider