Improvements to indexing in CQEngine

[codingchili]

If a field that is queried is of zero length, then an error will be thrown when the radix tree index is built.

• [ ] Support radix tree indexing attributes where values may be empty.
• [ ] Perform the initial indexing in a worker context.

[codingchili]

This has not been an issue for quite a while, closing for now.