streamingccl/streampb: ensure there's at least one go source file

When the directory is empty (prior to deriving .pb.go from .proto),
the go compiler has no package name for that directory.

This prevents the execution of `go mod tidy` and `make
vendor_rebuild`, and even prevents the generation of the `.pb.go`
file, resulting in a catch-22.

Release note: None

pkg/ccl/streamingccl/streampb/BUILD.bazel

@@ -32,6 +32,7 @@ go_proto_library(
 
 go_library(
     name = "streampb",
+    srcs = ["empty.go"],
     embed = [":streampb_go_proto"],
     importpath = "github.com/cockroachdb/cockroach/pkg/ccl/streamingccl/streampb",
     visibility = ["//visibility:public"],

pkg/ccl/streamingccl/streampb/empty.go

@@ -0,0 +1,11 @@
+// Copyright 2022 The Cockroach Authors.
+//
+// Licensed as a CockroachDB Enterprise file under the Cockroach Community
+// License (the "License"); you may not use this file except in compliance with
+// the License. You may obtain a copy of the License at
+//
+//     https://github.com/cockroachdb/cockroach/blob/master/licenses/CCL.txt
+
+package streampb
+
+// This file is intentionally left empty.