APEI: GHES: correctly return NULL for ghes_get_devices()

Since 315bada6 ("EDAC: Check for GHES preference in the
chipset-specific EDAC drivers"), vendor specific EDAC driver will not
probe correctly when CONFIG_ACPI_APEI_GHES is enabled but no GHES device
is present.  Make ghes_get_devices() return NULL when the GHES device
list is empty to fix the problem.

Fixes: 9057a3f7 ("EDAC/ghes: Prepare to make ghes_edac a proper module")
Signed-off-by: Li Yang <leoyang.li@nxp.com>
Reviewed-by: Tony Luck <tony.luck@intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>