Remove default parameter of UndoGroupHelper constructor

This can lead to mistakes. If no buffer is required at the beginning,
it is better to pass 'nullptr' explicitly.

Fix a case of missing parameter in GuiCompleter (introduced at
f3656600). This avoids some spurious "There is no group open (creating
one)" messages.

src/Undo.h

@@ -139,7 +139,7 @@ private:
  */
 class UndoGroupHelper {
 public:
-	UndoGroupHelper(Buffer * buf = 0);
+	UndoGroupHelper(Buffer * buf);
 
 	~UndoGroupHelper();
 

src/frontends/qt/GuiCompleter.cpp

@@ -687,7 +687,7 @@ void GuiCompleter::tab()
 	}
 
 	// Make undo possible
-	UndoGroupHelper ugh;
+	UndoGroupHelper ugh(cur.buffer());
 	cur.recordUndo();
 
 	// If completion is active, at least complete by one character

src/insets/InsetLabel.cpp

@@ -121,7 +121,7 @@ void InsetLabel::updateLabelAndRefs(docstring const & new_label,
 void InsetLabel::updateReferences(docstring const & old_label,
 		docstring const & new_label)
 {
-	UndoGroupHelper ugh;
+	UndoGroupHelper ugh(nullptr);
 	if (buffer().masterParams().track_changes) {
 		// With change tracking, we insert a new ref and
 		// delete the old one