For some reason, at least in the case of the "Test OpenCL" thing, we get here
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.)

Also add more useful (?) SAL_INFO calls in the area.

Change-Id: I1388786a3a0765af6eb01a63da31e5b83c7a616d