If you construct a QDir with a junk string, like this:

QString s = "junk";
QDir dir(s);
bool exists = dir.exists(); // false - correct

The exists() test will return false (correct).

But if you pass an empty string to the constructor, exists() will always return true;

 
QString s = "";
QDir dir(s);
bool exists = dir.exists(); // true - incorrect!

The workaround is to construct the QDir using the default constructor and use the setPath function to initialize it:

 
QString s = "";
QDir dir;
dir.setPath(s);
bool exists = dir.exists(); // false - correct