fix(common/core/desktop): Fix failing tests on armhf

This change fixes the FixupKeyboard method. In the .kmx files on
disk array pointers for an empty array point to the end of the file
(one byte after the end of the data structure). When we expand the
pointers in FixupKeyboard this leads to `cgp->dpKeyArray` to point to an
unallocated memory location - this shouldn't matter since we don't
try to read or write at this location if the Key array is empty,
but armhf still seems to raise a SIGBUS, probably because we cast
the value to a `LPKEY`. This change sets `cgp->dpKeyArray` to `NULL`
if we have an empty key array, similar to what's done for the strings.

Closes #5072.