[spring index] optimize empty array objects in internal index storage structure

[martinlippert]

Instead of creating new objects for empty arrays all the time (e.g. empty arrays for injection points), we should keep one empty array for injection points around and re-use that.

[martinlippert]

First step implemented for this optimization, but need to take a look at keeping this optimization when reading things from cache.

[martinlippert]

Closing now as completed.