Treat an array of null string pointers as no strings for OpenCL. For
some reason, at least in the case of the "Test OpenCL" thing, we got
an mpStringArray that is non-null but where all the elements
(rtl_uString pointers) in it are null. Treat that case as if the
mpStringArray was null.

This makes the tests "Test OpenCL" actually use OpenCL. Maybe it has other
useful effects, too. (But for normal spreadsheet use, the mpStringArray that
gets handled here *is* null when all the cells used by a formula group are
numbers. At least it seemed so in a simple test.)

Add a few more useful (?) SAL_INFO calls.

Some cosmetic changes to make the code look saner.

Return correct value from the OpenCL VLOOKUP implementation. Also,
when the lookup fails, produce the expected N/A error code instead of
a bare NaN.

Don't claim we support strings arguments in the OpenCL VLOOKUP.  The
string support certainly isn't complete or correct anyway. Partially
revert c3383aaf.

Add VLOOKUP to the set of opcodes that we trust the OpenCL
implementation for.

(cherry picked from commit 7e91726f)
(cherry picked from commit d4310b6c)
(cherry picked from commit 32b35d9d)
(cherry picked from commit 41a60940)
(cherry picked from commit 16226445)
(cherry picked from commit 7bb7539c)

Change-Id: I5402732c9ddcac1efcc25960a5c46175d7a90b4d
Reviewed-on: https://gerrit.libreoffice.org/18022Reviewed-by: Michael Meeks <michael.meeks@collabora.com>
Reviewed-by: Eike Rathke <erack@redhat.com>
Tested-by: Jan Holesovsky <kendy@collabora.com>