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/ShortcutManager.cpp

@@ -122,7 +122,7 @@ void ShortcutManager::OnChange(Base::Subject<const char*> &src, const char *reas
 
 void ShortcutManager::reset(const char *cmd)
 {
-    if (cmd && cmd[0]) {
+    if (!Base::Tools::isNullOrEmpty(cmd)) {
         QKeySequence oldShortcut = getShortcut(cmd);
         hShortcuts->RemoveASCII(cmd);
         if (oldShortcut != getShortcut(cmd))
@@ -172,7 +172,7 @@ QString ShortcutManager::getShortcut(const char *cmdName, const char *accel)
 
 void ShortcutManager::setShortcut(const char *cmdName, const char *accel)
 {
-    if (cmdName && cmdName[0]) {
+    if (!Base::Tools::isNullOrEmpty(cmdName)) {
         setTopPriority(cmdName);
         if (!accel)
             accel = "";