property_descr_get(): Fix a curious bug in the property() type: when

property_descr_get(): Fix a curious bug in the property() type: when
no get function was defined, the property's doc string was
inaccessible.  This was because the test for prop_get was made
*before* the test for a NULL/None object argument.

Also changed the property class defined in Python in a comment to test
for NULL to decide between get and delete; this makes it less Python
but then, assigning None to a property doesn't delete it!