fix wrong error when trying to instantiate abstract class

i don't understand all the implications but it seems the JavaAdapter will complain itself when it gets the wrong args.

without this change:

    js> new javax.swing.text.AbstractDocument()
    js: JavaAdapter requires at least one argument.

after:

    js> new javax.swing.text.AbstractDocument()
    js: error instantiating (0): class javax.swing.text.AbstractDocument is interface or abstract