If an ArrayHandleVirtual is constructed without an underlying concrete
array handle, then the Storage<T, StorageTagVirtual> holds a
StorageVirtual pointer that is null. Generally, a null
ArrayHandleVirtual cannot do much, but its operations should still be
correct. There were a few places where the Storage would blindly try to
use its StorageVirtual pointer without checking it first. This adds some
conditions that should correct the behavior when StorageVirtual is null.