radv: dont call calloc when BVH is empty

Usage of pointer returned by malloc(0) is UB.

Imho this part from radv_pipeline_cache.c is also suspicious:

static VkResult
radv_pipeline_cache_grow(struct radv_pipeline_cache *cache)
{
   const uint32_t table_size = cache->table_size * 2;
   const uint32_t old_table_size = cache->table_size;
   const size_t byte_size = table_size * sizeof(cache->hash_table[0]);
   struct cache_entry **table;
   struct cache_entry **old_table = cache->hash_table;

   table = malloc(byte_size);
   if (table == NULL)
      return vk_error(cache, VK_ERROR_OUT_OF_HOST_MEMORY);

Though I'm not sure what should be returned. (If this case can happen.)