radv: fix returning _Bool instead of pointer

When building for C23 the compiler warns about returning a boolean when
a different type is expected instead.

Change the code to return NULL instead of false, fixing the following
errors:

-----------------------------------------------------------------------
../src/amd/vulkan/radv_pipeline_cache.c: In function ‘radv_pipeline_cache_object_search’:
../src/amd/vulkan/radv_pipeline_cache.c:338:14: error: incompatible types when returning type ‘_Bool’ but ‘struct radv_pipeline_cache_object *’ was expected
  338 |       return false;
      |              ^~~~~
../src/amd/vulkan/radv_pipeline_cache.c:352:14: error: incompatible types when returning type ‘_Bool’ but ‘struct radv_pipeline_cache_object *’ was expected
  352 |       return false;
      |              ^~~~~
-----------------------------------------------------------------------

Reviewed-by: Faith Ekstrand <faith.ekstrand@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/36323>

src/amd/vulkan/radv_pipeline_cache.c

@@ -335,7 +335,7 @@ radv_pipeline_cache_object_search(struct radv_device *device, struct vk_pipeline
    *found_in_application_cache = false;
 
    if (radv_is_cache_disabled(device, cache))
-      return false;
+      return NULL;
 
    bool *found = found_in_application_cache;
    if (!cache) {
@@ -349,7 +349,7 @@ radv_pipeline_cache_object_search(struct radv_device *device, struct vk_pipeline
    radv_report_pso_cache_stats(device, pipeline, !!object);
 
    if (!object)
-      return false;
+      return NULL;
 
    return container_of(object, struct radv_pipeline_cache_object, base);
 }