Fixes #769: make processScroll add a loading node only when it runs a query

This is a regression from 6160a143d9f7b9ebdfe6b80b24cb1f4aa69f3cd7.  

This problem is not specific to the `JsonRest` store but it is easy to reproduce with that store.  Here are the prerequisites to trigger this bug:
 1. The grid's `loadingMessage` property needs to be set to a non-empty string.
 1. `result.total` returned from the store's `query` method needs to be null or undefined.

`processScroll` in `OnDemandList` was adding a loading node but then deciding to not run a query and continue the preload processing loop.  The code that removed the loading node was not running.  Before 6160a143d9f7b9ebdfe6b80b24cb1f4aa69f3cd7, the query would run with invalid start and count values and would usually not return any results.  `processScroll` would then remove the loading node in response to the lack of query results.