Fix Vulkan null_driver's EnumeratePhysicalDevices()

Counter to the Vulkan spec, the EnumeratePhysicalDevices() function was always
setting the count to 1, even when it didn't write anything.  This caused a
crash with the "dEQP-VK.api.info.instance.physical_devices" test case of the
Vulkan CTS v1.0.2.

Test: Run Vulkan CTS v1.0.2 (doesn't crash right away)
Change-Id: I1b7ba9f11229197efb825d8d0ad1b9ad88f029fb