Dont return an error for 0 physical device extensions

Dont return an error for 0 physical device extensions

Previously the logic of loader_add_device_extensions could return an error from
enumerating device extensions. This was because the code would return immediately
if either an error was returned from the driver or the count was zero. Now the
logic will return an error only if the driver returned an error. Zero device
extensions won't be treated as an error and will simply return VK_SUCCESS.