Generated FindFooByBar doesn't work well with null-filtered indices on spanner emulator

[JohnTitor]

Consider the following schema:

CREATE TABLE User (
  Foo STRING(MAX) NOT NULL,
  Bar STRING(MAX),
) PRIMARY KEY(Foo);

CREATE UNIQUE NULL_FILTERED INDEX User_Bar_index ON User(Bar);

Yo will generate a FindUserByBar fn looking at its unique index. But if we run tests against that fn on spanner emu, it will fail because of this:

absl::Status NullFilteredIndexUnusable(absl::string_view index_name) {
  return absl::Status(
      absl::StatusCode::kInvalidArgument,
      absl::Substitute(
          "The emulator is not able to determine whether the "
          "null filtered index $0 can be used to answer this query as it "
          "may filter out nulls that may be required to answer the query. "
          "Please test this query against Cloud Spanner. If you confirm "
          "against Cloud Spanner that the null filtered index can be used to "
          "answer the query, set the hint @{spanner_emulator."
          "disable_query_null_filtered_index_check=true} on the table "
          "to bypass this check in the emulator. This hint will be ignored by "
          "the production Cloud Spanner service and the emulator will accept "
          "the query and return a valid result when it is run with the check "
          "disabled.",
          index_name));
}

https://github.com/GoogleCloudPlatform/cloud-spanner-emulator/blob/4bc1ce05ee511713318e991fbd076b884ea404cc/common/errors.cc#L1788-L1804

A simple way to make it work would be passing spanner_emulator.disable_query_null_filtered_index_check=true to the emu but AFAIK there's no option on yo to do that. Maybe custom template would be a workaround here but It'd be great if upstream has a fix.