message: Check if environment variables are null before assigning to string
diff: @@ -94,8 +94,16 @@ void OptimizeDatasetOp::MakeDataset(OpKernelContext* ctx, DatasetBase* input,
  // This is currently empty; we have no live experiments yet.
  absl::flat_hash_map<string, uint64> live_experiments;
  - const string opt_ins_raw = std::getenv("TF_DATA_EXPERIMENT_OPT_IN");
  - const string opt_outs_raw = std::getenv("TF_DATA_EXPERIMENT_OPT_OUT");
  + const char* opt_ins_raw_cs = std::getenv("TF_DATA_EXPERIMENT_OPT_IN");
  + const char* opt_outs_raw_cs = std::getenv("TF_DATA_EXPERIMENT_OPT_OUT");
  + string opt_ins_raw;
  + if (opt_ins_raw_cs != nullptr) {
  + opt_ins_raw = string(opt_ins_raw_cs);
  + }
  + string opt_outs_raw;
  + if (opt_outs_raw_cs != nullptr) {
  + opt_outs_raw = string(opt_outs_raw_cs);
  + }
  auto hash_func = [](const string& str) { return Hash64(str); };
  optimizations = SelectOptimizations(
  job_name, opt_ins_raw, opt_outs_raw, live_experiments,
  