Code review comment: this should just be set to the default value now.

Code under review:
FirestoreStub getFirestoreStub(PipelineOptions options) {
    try {
      FirestoreSettings.Builder builder =
          FirestoreSettings.newBuilder()
              .setHeaderProvider(
                  new FixedHeaderProvider() {
                    @Override
                    public Map<@NonNull String, @NonNull String> getHeaders() {
                      return ImmutableMap.of("User-Agent", options.getUserAgent());
                    }
                  });

      RetrySettings retrySettings = RetrySettings.newBuilder().setMaxAttempts(1).build();

      builder.applyToAllUnaryMethods(
          b -> {
            b.setRetrySettings(retrySettings);
            return null;
          });

      FirestoreOptions firestoreOptions = options.as(FirestoreOptions.class);
      String emulatorHostPort = firestoreOptions.getEmulatorHost();
      if (emulatorHostPort == null) {
        emulatorHostPort = System.getenv(FIRESTORE_EMULATOR_HOST_ENV_VARIABLE);
      }
      if (emulatorHostPort != null) {
        builder
            .setCredentialsProvider(FixedCredentialsProvider.create(new EmulatorCredentials()))
            .setEndpoint(emulatorHostPort)
            .setTransportChannelProvider(
                InstantiatingGrpcChannelProvider.newBuilder()
                    .setEndpoint(emulatorHostPort)
                    .setChannelConfigurator(c -> c.usePlaintext())
                    .build());
      } else {
        GcpOptions gcpOptions = options.as(GcpOptions.class);
        String host = firestoreOptions.getHost();
        if (host == null) {
          host = System.getenv().getOrDefault(FIRESTORE_HOST_ENV_VARIABLE, DEFAULT_FIRESTORE_HOST);
        }
        builder
            .setCredentialsProvider(FixedCredentialsProvider.create(gcpOptions.getGcpCredential()))
            .setEndpoint(host);
      }

      ClientContext clientContext = ClientContext.create(builder.build());
      return GrpcFirestoreStub.create(clientContext);
    } catch (Exception e) {
      throw new RuntimeException(e);
    }
  }

Line commented on: host = System.getenv().getOrDefault(FIRESTORE_HOST_ENV_VARIABLE, DEFAULT_FIRESTORE_HOST);

Revised code:
FirestoreStub getFirestoreStub(PipelineOptions options) {
    try {
      FirestoreSettings.Builder builder =
          FirestoreSettings.newBuilder()
              .setHeaderProvider(
                  new FixedHeaderProvider() {
                    @Override
                    public Map<@NonNull String, @NonNull String> getHeaders() {
                      return ImmutableMap.of("User-Agent", options.getUserAgent());
                    }
                  });

      RetrySettings retrySettings = RetrySettings.newBuilder().setMaxAttempts(1).build();

      builder.applyToAllUnaryMethods(
          b -> {
            b.setRetrySettings(retrySettings);
            return null;
          });

      FirestoreOptions firestoreOptions = options.as(FirestoreOptions.class);
      String emulatorHostPort = firestoreOptions.getEmulatorHost();
      if (emulatorHostPort != null) {
        builder
            .setCredentialsProvider(FixedCredentialsProvider.create(new EmulatorCredentials()))
            .setEndpoint(emulatorHostPort)
            .setTransportChannelProvider(
                InstantiatingGrpcChannelProvider.newBuilder()
                    .setEndpoint(emulatorHostPort)
                    .setChannelConfigurator(c -> c.usePlaintext())
                    .build());
      } else {
        GcpOptions gcpOptions = options.as(GcpOptions.class);
        builder
            .setCredentialsProvider(FixedCredentialsProvider.create(gcpOptions.getGcpCredential()))
            .setEndpoint(firestoreOptions.getHost());
      }

      ClientContext clientContext = ClientContext.create(builder.build());
      return GrpcFirestoreStub.create(clientContext);
    } catch (Exception e) {
      throw new RuntimeException(e);
    }
  }