Cursor no-next-reason appears not to be forwarded through scans on `BackingLocatableResolverStore` result sets

[alecgrieser]

I'm not sure what the exact perimeter of this is, but scans of the types in a LocatableResolverBackedStore appear not to be forwarding their no-next-reason. In particular, when someone issues executeScan on scan of one of those stores, then when the scan reaches the end of its run, rather than returning a continuation with a "real" reason, we get null back. For example, the scans in the BackingLocatableResolverStoreTest all return null instead of a real reason.

It's possible that this is a wider problem (for example, it may also apply to BackingRecordStores), but I encountered this while writing tests of that store (see #3221).