Base: Add isNullOrEmpty string helper

This adds isNullOrEmpty string helper that cheks if string is... well
null or empty. It is done to improve readability of the code and better
express intent.

src/Gui/Document.cpp

@@ -1701,7 +1701,7 @@ void Document::RestoreDocFile(Base::Reader &reader)
         localreader->readElement("Camera");
         const char* ppReturn = localreader->getAttribute("settings");
         cameraSettings.clear();
-        if(ppReturn && ppReturn[0]) {
+        if(!Base::Tools::isNullOrEmpty(ppReturn)) {
             saveCameraSettings(ppReturn);
             try {
                 const char** pReturnIgnore=nullptr;