Fix a segfault in client_lb_end2end_test (#29568)

In the SubchannelStreamClient constructor, do not initialize an
absl::string_view with a null pointer; this leads to strlen() being
called on the null pointer. Let the absl::string_view be initialized
with the string literal "SubchannelStreamClient" in this case instead.

Fixes #29567.

src/core/ext/filters/client_channel/subchannel_stream_client.cc

@@ -50,7 +50,8 @@ SubchannelStreamClient::SubchannelStreamClient(
       call_allocator_(
           ResourceQuotaFromChannelArgs(connected_subchannel_->args())
               ->memory_quota()
-              ->CreateMemoryAllocator(tracer)),
+              ->CreateMemoryAllocator(
+                  (tracer != nullptr) ? tracer : "SubchannelStreamClient")),
       event_handler_(std::move(event_handler)),
       retry_backoff_(
           BackOff::Options()