bindingsEngine.containsKey throws Exception when key is not found

If no variable called `ten` is defined, the following Scala code throws `java.lang.NullPointerException`:
```
new ScriptEngineManager()
  .getEngineByName("Java")
  .getContext
  .getBindings(ScriptContext.ENGINE_SCOPE)
  .containsKey("ten")
```

Similar code works for `scripting-scala`. However, this bug can is also present for `scripting-clojure`.