[infra] Fix build cursor when there're no more builds.
[infra] Fix build cursor when there're no more builds.

There is a case where the API returns a cursor for the next set of results,
but the next page of results is empty when requested and the builds array
is absent. This change adds support for that case and avoids a crash.

Change-Id: I56b419990617ea407c05abb561eeb969f49ecb5f
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/121220
Reviewed-by: Alexander Thomas <29642742b6693024c89de8232f2e2542cf7eedf7@google.com>

dart-archive/dart-sdk,dart-lang/sdk,dartino/dart-sdk,dartino/dart-sdk,dart-lang/sdk,dartino/dart-sdk,dart-lang/sdk,dart-archive/dart-sdk,dart-lang/sdk,dart-archive/dart-sdk,dart-archive/dart-sdk,dart-lang/sdk,dart-archive/dart-sdk,dart-archive/dart-sdk,dartino/dart-sdk,dart-lang/sdk,dartino/dart-sdk,dart-archive/dart-sdk,dart-archive/dart-sdk,dart-lang/sdk,dart-lang/sdk,dartino/dart-sdk,dart-archive/dart-sdk,dartino/dart-sdk,dartino/dart-sdk,dartino/dart-sdk