results are in the range of first_iterator to last_iterator.
If one of those is nullptr the algorithm would fail anyway.

This removes some impossible checks in
sw/source/core/unocore/unochart.cxx:
SwChartDataProvider::detectArguments.
A sorted range still holds the same values and has
the same length as the original range.
Replacing raw pointers eases reading this code.

Change-Id: If96bd11e9167488346a57e9e08507ac42338d3cd
Reviewed-on: https://gerrit.libreoffice.org/39683Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Noel Grandin <noel.grandin@collabora.co.uk>