Allow nullptr when logging ToSinkAlso()

[jiawen]

Our project has a debugging mechanism whereby based on a config option, we may want to also log to an additional, independent absl::LogSink*. However ToSinkAlso() does not accept nullptr as a parameter - this is reasonable but forces us to use somewhat unsightly workarounds:

Either:

void MyFunc(absl::LogSink* optional_sink) {
if (optional_sink) {
  LOG(INFO).ToSinkAlso(optional_sink) << "my message";
} else {
  LOG(INFO) << "my message";
}

or:

define a class NoopLogSink : public absl::LogSink whose Send() does nothing and has a global singleton. Then the code looks like:

void MyFunc(absl::LogSink* optional_sink = NoopLogSink::singleton()) {
  LOG(INFO).ToSinkAlso(optional_sink) << "my message";

Is there a less ugly alternative that I'm missing? The only thing I can think of is to write a wrapper macro around all the LOG ones but it's a pretty big surface.

[derekmauro]

It might be possible to put the logic to decide whether to log directly into the sink that is currently optional.