Add test for stepSize=null

This was recently fixed in https://github.com/scijava/scijava-ui-swing/commit/40bd474d8b9b169d521a621ddf9772a718ce6e26 (thanks @ctrueden !), but it's not yet part of any unit test (I failed to test for it in `SwingNumberWidgetTest` added in https://github.com/scijava/scijava-ui-swing/commit/d89b7c959ba86d8cc37d3cc9ca1a6416d997fd90). Let's add a test to prevent future regressions.