use empty() instead of comparison with an empty string

rtgui/darkframe.cc

@@ -125,7 +125,7 @@ void DarkFrame::read(const rtengine::procparams::ProcParams* pp, const ParamsEdi
         Glib::ustring fname = Glib::path_get_basename(dfp->GetCurrentImageFilePath());
         Glib::ustring filetype;
 
-        if (fname != "") {
+        if (!fname.empty()) {
             // get image filetype, set filter to the same as current image's filetype
             std::string::size_type idx;
             idx = fname.rfind('.');