In the case where a window has not been assigned a title, it does not have a title and I suspect that is why GetWindow with the title keyword sets S_Value to "". When a window does not have a title, Igor displays the name of the graph followed by a ":", but internally I don't believe that Igor considers that to be the title of the window. But you should be able to do something like this:

Function/S ConstructTitle(wName)
	String wName		// The name of the window.
	String wTitle
 
	GetWindow $(wName) title
	if (strlen(S_value) <= 0)
		wTitle = wName + ":"
	else
		wTitle = S_value
	endif
	return wTitle
End