Handle GSS_C_NO_OID_SET when creating sets

[DirectXMan12]

Some methods can return GSS_C_NO_OID_SET on success, so we should handle
that in our set converter by returning the empty set.

Fixes #148

[DirectXMan12]

cc @simo5 @frozencemetery can one of you test this, and or figure out a good way to write a gated unit test for it?

[frozencemetery]

>>> import gssapi
>>> ntlm_mech = gssapi.OID.from_int_seq("1.3.6.1.4.1.311.2.2.10")
>>> gssapi.raw.inquire_attrs_for_mech(ntlm_mech)
InquireAttrsResult(mech_attrs=set([]), known_mech_attrs=set([]))
>>> 

[simo5]

One nit but otherwise LGTM

[simo5]

If we ever need the distinction we may need to make an incompatible API change and start returning None, is that ok ?

[DirectXMan12]

yes, but we'll have to rev the major version. I'm curious as to what that situation might be -- you're not using this to detect errors (we have errors for that), so fundamentally, what's the difference here between "a set with no items", and "no set" from the user perspective.

[simo5]

ok

[frozencemetery]

@DirectXMan12 are you happy with the way this is being tested, or do you want something different?

[DirectXMan12]

Yeah, I'm not certain there's much more we can do, unless there's a dummy OID that we know will never have any attrs. This should be good to merge.