fix NPE in blank 3d view; no sample = no notifications

[olbris]

Added a check for no sample to prevent NPE when interacting with (scroll wheel) a blank 3d window.

Fixes JW-52721. Also reported by AIND in Slack.

@porterbot, you can merge when you approve.

@stuarteberg

[olbris]

Relevant part of traceback from JW-52721:

Stack Trace:
java.lang.NullPointerException
at org.janelia.scenewindow.SceneInteractor.notifyObservers(SceneInteractor.java:49)
at org.janelia.scenewindow.OrbitPanZoomInteractor.mouseDragged(OrbitPanZoomInteractor.java:178)
at org.janelia.horta.HortaMouseEventDispatcher.mouseDragged(HortaMouseEventDispatcher.java:79)
at org.janelia.workstation.controller.listener.TolerantMouseClickListener.mouseDragged(TolerantMouseClickListener.java:100)
at java.awt.Component.processMouseMotionEvent(Component.java:6587)
at java.awt.Component.processEvent(Component.java:6308)

[porterbot]

Do think we also need a notification to the user when they try to open the Horta 3D view without a sample (clicking on the Horta 3D checkbox)?

[olbris]

I think it's fine without a notification. The window opens up black. No fields in the control center will be populated.