[BUGFIX] Ensure record set integrity

When pre-processors are called some items in the data record set
may have been unset. Further steps in the external import process
rely on the record set being a continuously indexed array.
This situation must be enforced after each pre-processor call.

Resolves: #56186
Releases: 2.3
Change-Id: Id1dc8ef0dfadded74ec39c792412dc41d4a2fb5d
Reviewed-on: https://review.typo3.org/30503
Reviewed-by: Francois Suter
Tested-by: Francois Suter