Fix potential nullptr names in resource_user/slice_allocator creation (#27154)

absl::string_view permits construction with nullptr, which yields UB.
This PR fixes all call sites that could have created these objects with
nullptr names.

src/core/ext/transport/chttp2/transport/chttp2_slice_allocator.cc

@@ -59,7 +59,7 @@ Chttp2SliceAllocatorFactory::~Chttp2SliceAllocatorFactory() {
 std::unique_ptr<SliceAllocator>
 Chttp2SliceAllocatorFactory::CreateSliceAllocator(absl::string_view peer_name) {
   return absl::make_unique<Chttp2SliceAllocator>(
-      grpc_resource_user_create(resource_quota_, peer_name.data()));
+      grpc_resource_user_create(resource_quota_, peer_name));
 }
 
 }  // namespace experimental

src/core/lib/iomgr/resource_quota.cc

@@ -804,7 +804,7 @@ grpc_resource_user* grpc_resource_user_create(
   for (int i = 0; i < GRPC_RULIST_COUNT; i++) {
     resource_user->links[i].next = resource_user->links[i].prev = nullptr;
   }
-  if (name != nullptr) {
+  if (!name.empty()) {
     resource_user->name = std::string(name);
   } else {
     resource_user->name = absl::StrCat(

src/cpp/thread_manager/thread_manager.cc

@@ -58,7 +58,8 @@ ThreadManager::ThreadManager(const char* name,
       max_pollers_(max_pollers == -1 ? INT_MAX : max_pollers),
       num_threads_(0),
       max_active_threads_sofar_(0) {
-  resource_user_ = grpc_resource_user_create(resource_quota, name);
+  resource_user_ =
+      grpc_resource_user_create(resource_quota, name != nullptr ? name : "");
 }
 
 ThreadManager::~ThreadManager() {