FIX: return builtin CuVSProvider when mock is null in internalClusterTests (#138155) (#138215)

In #137126 we refactored GPUSupport to use static initialization. Some IT tests were fixed to support this, but we did not notice that GPUIndexIT needed attention too.
GPUIndexIT uses the real (builtin) CuVSProvider, and thus does not need to provide a mock.

This PR changes the Test CuVSProvider to "bypass" itself when we do no specify a mock, so that in those cases we use the builtin provider.

Author: ldematte

x-pack/plugin/gpu/src/internalClusterTest/java/org/elasticsearch/plugin/gpu/TestCuVSServiceProvider.java

@@ -25,6 +25,9 @@ public class TestCuVSServiceProvider extends CuVSServiceProvider {
 
     @Override
     public CuVSProvider get(CuVSProvider builtin) {
+        if (mockedGPUInfoProvider == null) {
+            return builtin;
+        }
         return new CuVSProviderDelegate(builtin) {
             @Override
             public GPUInfoProvider gpuInfoProvider() {