fix NPE on (tg/open?) when no graph open or graph is shutdown


calling `tg/open?` before `tg/open` ever:
```
=> (tg/open?)
IllegalArgumentException No matching field found: isOpen for class clojure.lang.Var$Unbound  clojure.lang.Reflector.getInstanceField (Reflector.java:271)
```

and after `tg/shutdown` : 
```
=> (tg/open?)
NullPointerException   clojure.lang.Reflector.invokeNoArgInstanceMember (Reflector.java:296)
```