[UR][OpenCL] Fix handling of urPlatformGet where there are no platforms (#18076)

The logic here was incorrect in cases where no platforms are available
(and thus `clGetPlatformIDs` returns `CL_PLATFORM_NOT_FOUND_KHR`).

Author: RossBrunton

unified-runtime/source/adapters/opencl/platform.cpp

@@ -82,26 +82,19 @@ urPlatformGet(ur_adapter_handle_t, uint32_t NumEntries,
       uint32_t NumPlatforms = 0;
       cl_int Res = clGetPlatformIDs(0, nullptr, &NumPlatforms);
 
-      std::vector<cl_platform_id> CLPlatforms(NumPlatforms);
-      Res = clGetPlatformIDs(static_cast<cl_uint>(NumPlatforms),
-                             CLPlatforms.data(), nullptr);
-
-      /* Absorb the CL_PLATFORM_NOT_FOUND_KHR and just return 0 in num_platforms
-       */
-      if (Res == CL_PLATFORM_NOT_FOUND_KHR) {
+      if (NumPlatforms == 0 || Res == CL_PLATFORM_NOT_FOUND_KHR) {
         if (pNumPlatforms) {
           *pNumPlatforms = 0;
-          return UR_RESULT_SUCCESS;
         }
-      }
-      /* INVALID_VALUE is returned when the size is invalid, special case it
-       * here
-       */
-      if (Res == CL_INVALID_VALUE && phPlatforms != nullptr &&
-          NumEntries == 0) {
-        return UR_RESULT_ERROR_INVALID_SIZE;
+        return UR_RESULT_SUCCESS;
       }
       CL_RETURN_ON_FAILURE(Res);
+
+      std::vector<cl_platform_id> CLPlatforms(NumPlatforms);
+      Res = clGetPlatformIDs(static_cast<cl_uint>(NumPlatforms),
+                             CLPlatforms.data(), nullptr);
+      CL_RETURN_ON_FAILURE(Res);
+
       try {
         for (uint32_t i = 0; i < NumPlatforms; i++) {
           auto URPlatform =