Alter jedi.Interpreter() namespace argument default from [] to [{}]

A few lines into the constructor, an `Interpreter` calls

        # Here we add the namespaces to the current parser.
        interpreter.create(self._evaluator, namespaces[0], self._parser.module())

The default value for the `namespaces` parameter currently is `[]`, such that an exception is thrown. I've changed that default to `[{}]`.