Prefer using Path.resolve() for path construction over string concatenation

Is it really a return we want if columns is empty? Maybe a continue is better to not interrupt the writing process for next datasets.

I am afraid that we could have performance issues here. If i am not mistaken Files.readString() will put all the content of the file in RAM, which can be a problem for big files.

Suggestion : deport this logic in a method where we use stream to read line by line the file, something like :

Path tempFile = Files.createTempFile("csv_clean_", ".tmp");

try (BufferedReader reader = Files.newBufferedReader(file);
     BufferedWriter writer = Files.newBufferedWriter(tempFile, StandardOpenOption.WRITE)) {

    String line;
    while ((line = reader.readLine()) != null) {
        writer.write(line.replace(NULL_MARKER, "\"\""));
        writer.newLine();
    }
}

Files.move(tempFile, file, java.nio.file.StandardCopyOption.REPLACE_EXISTING);