Drop NULL from named win32 handle description.

Address review comments from Ben to remove NULL
from named win32 handle description.

Fixes #943

api/opencl_runtime_layer.asciidoc

@@ -5621,7 +5621,7 @@ buffer or an image memory object from an external handle:
  * {CL_EXTERNAL_MEMORY_HANDLE_OPAQUE_WIN32_NAME_KHR_anchor} specifies an NT handle name that
  has only limited valid usage outside of OpenCL and other compatible
  APIs.
- NT handle name is NULL or a null-terminated UTF-16 string naming the payload to import.
+ NT handle name is a null-terminated UTF-16 string naming the payload to import.
  It must be compatible with the functions `DuplicateHandle`,
  `CloseHandle`, `CompareObjectHandles`, `GetHandleInformation`, and
  `SetHandleInformation`.
@@ -13139,7 +13139,7 @@ a semaphore from an external handle:
  when all semaphore objects associated with it are destroyed.
  * {CL_SEMAPHORE_HANDLE_OPAQUE_WIN32_NAME_KHR} specifies an NT handle name that has
  only limited valid usage outside of OpenCL and other compatible APIs.
- NT handle name is NULL or a null-terminated UTF-16 string naming the payload to import.
+ NT handle name is a null-terminated UTF-16 string naming the payload to import.
  It must be compatible with the functions `DuplicateHandle`,
  `CloseHandle`, `CompareObjectHandles`, `GetHandleInformation`, and
  `SetHandleInformation`.