fix stalling on PagedCollection, when value is empty but nextLink is valid

[weidongxu-microsoft]

Fix #996

[yaohaizh]

@weidongxu-microsoft I'm okay with this change. But my second taking is that whether the KeyVault team breaks the protocol of the swagger spec for nextLinkPage because empty for next link is definitely not a good practice.

https://github.com/Azure/autorest/blob/master/docs/extensions/readme.md#x-ms-pageable

//cc @yungezz

[weidongxu-microsoft]

@yaohaizh

Keyvault actually have an empty value, with valid nextLink. I.e., they seems got one page which is empty, and still need to continue the paging.

My guess is they are doing some filtering internally on the page result before returning to us.

@jasonxdhu

Would you please share more on this case?

[yungezz]

agree with @yaohaizh , other sdks should meet similar issue: nextLInk valid while empty page content. Can we have a look at Keyvault swagger and file issue there if indeed a swagger issue?