ImageJ launcher: avoid NullPointerException
ImageJ launcher: avoid NullPointerException

The issue was caused by attempting to System.setProperty with a null
value. We now check for that.

Debugging was a little tricky because the error message was not printed
until DetachCurrentThread was called... so not calling
DetachCurrentThread appeared to fix the issue. But it actually only hid
the error message.

Cherry-picked from fiji.git.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

This used to be revision r5200.

biovoxxel/imagej,TehSAUCE/imagej,biovoxxel/imagej,TehSAUCE/imagej,biovoxxel/imagej,TehSAUCE/imagej