fix: An empty string `nextMarker` should be treated as end-of-paging.

[potrebic]

The Box V2 API docs are inconsistent on this. However it is stated on the paging docs(1):

If this value is null or an empty string there are no more results to fetch.

So with this change an empty string next_marker will end paging.

(1): https://developer.box.com/guides/api-calls/pagination/marker-based/#collections

Issue Link :link:

https://github.com/box/box-ios-sdk/issues/892

[coveralls]

Coverage: 88.607% (+0.003%) from 88.604% when pulling 7293c4c23adf903b11fa43c007bb68efaff22d12 on potrebic:ppotrebic/empty-marker into ffff13c1e77a8e09e888a9f264b27995318e2d1d on box:main.