Perform a None -> null conversion in JLineConsole.setStartupHook()

JLineConsole.readerReadLine() checks if startupHook is null before invoking __call__(). If startupHook is None rather than null, the invocation will proceed, resulting in a TypeError.

This fix converts None to null in JLineConsole.setStartupHook() allowing the null-check in readerReadLine() to be sufficient. Put the check in setStartupHook since it is likely to be called less often.

This fix allows the basic IPython console to work on Jython, with the following Jython specific modifications:

[https://github.com/Brittix1023/ipython/tree/basic-jython-fixes](https://github.com/Brittix1023/ipython/tree/basic-jython-fixes)