[wpiutil] DataLogBackgroundWriter: Normalize empty path name (#7151)

An empty path isn't valid on it's own, so fs::space always returns an error. This results in UINT_MAX bytes being used instead of the actual free space, which means a default constructed DataLogBackgroundWriter won't stop for low space.

Using "." instead makes the directory path the current working directory, which is the desired behavior

wpiutil/src/main/native/cpp/DataLogBackgroundWriter.cpp

@@ -176,7 +176,8 @@ static std::string MakeRandomFilename() {
 }
 
 struct DataLogBackgroundWriter::WriterThreadState {
-  explicit WriterThreadState(std::string_view dir) : dirPath{dir} {}
+  explicit WriterThreadState(std::string_view dir)
+      : dirPath{dir.empty() ? "." : dir} {}
   WriterThreadState(const WriterThreadState&) = delete;
   WriterThreadState& operator=(const WriterThreadState&) = delete;
   ~WriterThreadState() { Close(); }