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/App/PropertyFile.cpp

@@ -29,6 +29,7 @@
 #include <Base/Stream.h>
 #include <Base/Writer.h>
 #include <Base/Uuid.h>
+#include <Base/Tools.h>
 
 #include "PropertyFile.h"
 #include "Document.h"
@@ -110,7 +111,7 @@ std::string PropertyFileIncluded::getOriginalFileName() const
 
 void PropertyFileIncluded::setValue(const char* sFile, const char* sName)
 {
-    if (sFile && sFile[0] != '\0') {
+    if (!Base::Tools::isNullOrEmpty(sFile)) {
         if (_cValue == sFile) {
             throw Base::FileSystemError("Not possible to set the same file!");
         }