New name UI_UPDATE for interpret_move's return "".

Now midend.c directly tests the returned pointer for equality to this
value, instead of checking whether it's the empty string.

A minor effect of this is that games may now return a dynamically
allocated empty string from interpret_move() and treat it as just
another legal move description. But I don't expect anyone to be
perverse enough to actually do that! The main purpose is that it
avoids returning a string literal from a function whose return type is
a pointer to _non-const_ char, i.e. we are now one step closer to
being able to make this code base clean under -Wwrite-strings.

dominosa.c

@@ -1119,7 +1119,7 @@ static char *interpret_move(const game_state *state, game_ui *ui,
 
         move_cursor(button, &ui->cur_x, &ui->cur_y, 2*w-1, 2*h-1, 0);
 
-	return "";
+	return UI_UPDATE;
     } else if (IS_CURSOR_SELECT(button)) {
         int d1, d2;
 
@@ -1152,7 +1152,7 @@ static char *interpret_move(const game_state *state, game_ui *ui,
         } else {
             return NULL;
         }
-        return "";
+        return UI_UPDATE;
     }
 
     return NULL;