[cpp] rename is_finished to produced_empty_tree in cpp and R code

[StrikerRUS]

Refer to #6890 (comment).

I did not want to touch R and C code and docs. Consequently the name and doc of the return value is inconsistent between the python, C, and R code and docs.

[StrikerRUS]

I don't think that it worth introducing a breaking change for renaming this param. I believe that changing the description is enough.

[jameslamb]

Apologies if I am showing my ignorance here, but I don't think that renaming the argument to a C function is an API-breaking or ABI-breaking change, because C functions only accept positional arguments.

I don't think renaming is_finished to produced_empty_tree here would break any existing code.

[StrikerRUS]

Thanks a lot for your early feedback!
You're totally right! Pushed changes in f16671c.

Now the only left place with is_finished is

LightGBM/src/boosting/gbdt.cpp

Lines 239 to 245 in 4c37695

	bool is_finished = false;
	auto start_time = std::chrono::steady_clock::now();
	for (int iter = 0; iter < config_->num_iterations && !is_finished; ++iter) {
	is_finished = TrainOneIter(nullptr, nullptr);
	if (!is_finished) {
	is_finished = EvalAndCheckEarlyStopping();
	}

I think it's correct naming because this variable is used as a return value not only from TrainOneIter() but also from EvalAndCheckEarlyStopping().