FIX: failed to remove policy from non-filled casbin model.

[DarcJC]

If someone using model like the RBAC model that not fill 6 slot(v0-v5), while adding the policy to the database(I'm testing with postgresql) the empty slot will be treat as NULL.
In adapter.py#L63 and adapter.py#L73 are constructing the query with ""(empty string) but None(NULL). It will never match any records in this situation, so policies will never be deleted(But casbin will delete them from its in-memory model which will break the consistency.)

model e.g.

_model = """
[request_definition]
r = sub, obj, act

[policy_definition]
p = sub, obj, act, eft

[role_definition]
g = _, _

[policy_effect]
e = some(where (p.eft == allow)) && !some(where (p.eft == deny))

[matchers]
m = g(r.sub, p.sub) && keyMatch(r.obj, p.obj) && keyMatch(r.act, p.act)
"""

[hsluoyz]

@thearchitector

[thearchitector]

@hsluoyz @DarcJC Thanks for bringing this to my attention! Closing in favor of 58444d9, to fix the type check.