message: Fix incorrect bigtable check.
diff: @@ -338,7 +338,7 @@ class SelfPlayer {
  std::vector<std::string> bigtable_spec =
  absl::StrSplit(FLAGS_output_bigtable, ',');
  bool use_bigtable = bigtable_spec.size() == 3;
  - if (!bigtable_spec.empty() && !use_bigtable) {
  + if (!FLAGS_output_bigtable.empty() && !use_bigtable) {
  MG_FATAL()
  << "Bigtable output must be of the form: project,instance,table";
  return;
  