Fixed passing NULL array from get_instance

I have a python provider and I want to return an instance with NULL value of
string[] (or any other array) property.

Therefore I have in my get_instance method:
model['MyProperty'] = pywbem.CIMProperty(
	name='MyProperty',
	value=None,
	type='string',
	array_size=0,
	is_array=True)

Under Pegasus CIMOM I get traceback, because Pegasus pedantically checks
property types;
  File "/usr/lib/python2.7/site-packages/cmpi_pywbem_bindings.py", line 761, in pywbem2cmpi_inst
      cinst.set_property(str(prop.name), data, ctype)
  File "/usr/lib/python2.7/site-packages/cmpi.py", line 809, in set_property
      return _cmpi.CMPIInstance_set_property(self, *args)
  CIMError: (1, '61')

This patch adds two checks for NULL value of array and makes sure that the
CIMOM really gets NULL value of the property.