Without this testFormulaRefUpdateDeleteAndShiftLeft2 fails with OpenCL
enabled (and the mnOpenCLMinimumFormulaGroupSize check disabled).
If I'm getting it right, anything working with #REF! (=invalid reference)
should result in #REF! too, but without this fix the function effectively
converted the #REF! to an empty cell.

Change-Id: Icdda64ad751d7c0e9c0d5995673e4fb30c4c9ac4
Reviewed-on: https://gerrit.libreoffice.org/55672Reviewed-by: Michael Meeks <michael.meeks@collabora.com>
Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Tor Lillqvist <tml@collabora.com>