Ensure that vector array has a numeric array of NaN's for empty range.

With this change, we ensure that mpNumArray is never NULL even when the
range consists entirely of empty cells.  For an empty range, mpNumArray
will be non-NULL and filled with NaN's while mpStrArray will be NULL.

P.S. We need to backport this because without this, the existing OpenCL
code could lead to a crash during OpenCL enabled calculations.

(cherry picked from commit 2ec3127da35933fc6d5ac47ecedd0267f67c1d62)

Change-Id: If5cead26ebe917af150cf7e39e17afe3f310beb7
Reviewed-on: https://gerrit.libreoffice.org/8021
Tested-by: Markus Mohrhard <markus.mohrhard@googlemail.com>
Reviewed-by: Markus Mohrhard <markus.mohrhard@googlemail.com>