Fix null pointer dereference in tsl::profiler::CreateStub
The bug was found by Svace static analyzer:

1. channel may be null
2. it is passed to grpc::ProfileAnalysis::NewStub(channel)
3. then passed to grpc::ProfileAnalysis::Stub::Stub(channel)
4. then constructor grpc::internal::RpcMethod::RpcMethod() dereferences channel via channel->RegisterMethod(name)

cc @mihaimaruseac