fix auto paginator when argument last is null

[sukano]

Q	A
Bug fix?	yes
New feature?	no
BC breaks?	no
Deprecations?	no
Tests pass?	yes
Documented?	no
Fixed tickets	no
License	MIT

Fix Paginator.auto() when $args['last'] === null (it try to backward and failed).

[mcg-web]

this issue is cause by this line using isset instead of array_key_exists since array_key_exists return true for an existing key even the value is null.

[mcg-web]

Thank you @sukano 👍