Spatial remove "null" from controls on read(...).

It's possible if the class of the control is not found (may be deleted).

In SDK with this modification, the exception about ClassNotFound is "notify"
but it no longer raise a NPE (on clone()) and the j3o can be open and edited
without the missing Control.

```
SEVERE [com.jme3.export.binary.BinaryImporter]: Exception
java.lang.ClassNotFoundException: vdrones.NewControl
	at java.net.URLClassLoader$1.run(URLClassLoader.java:372)
	at java.net.URLClassLoader$1.run(URLClassLoader.java:361)
	at java.security.AccessController.doPrivileged(Native Method)
	at java.net.URLClassLoader.findClass(URLClassLoader.java:360)
	at java.lang.ClassLoader.loadClass(ClassLoader.java:424)
	at java.lang.ClassLoader.loadClass(ClassLoader.java:357)
	at com.jme3.export.SavableClassUtil.fromName(SavableClassUtil.java:195)
[catch] at com.jme3.export.binary.BinaryImporter.readObject(BinaryImporter.java:333)
	at com.jme3.export.binary.BinaryInputCapsule.resolveIDs(BinaryInputCapsule.java:483)
	at com.jme3.export.binary.BinaryInputCapsule.readSavableArray(BinaryInputCapsule.java:471)
	at com.jme3.export.binary.BinaryInputCapsule.readSavableArrayList(BinaryInputCapsule.java:587)
	at com.jme3.scene.Spatial.read(Spatial.java:1333)
	at com.jme3.scene.Node.read(Node.java:608)
	at com.jme3.export.binary.BinaryImporter.readObject(BinaryImporter.java:344)
	at com.jme3.export.binary.BinaryInputCapsule.resolveIDs(BinaryInputCapsule.java:483)
	at com.jme3.export.binary.BinaryInputCapsule.readSavableArray(BinaryInputCapsule.java:471)
	at com.jme3.export.binary.BinaryInputCapsule.readSavableArrayList(BinaryInputCapsule.java:587)
	at com.jme3.scene.Node.read(Node.java:598)
	at com.jme3.export.binary.BinaryImporter.readObject(BinaryImporter.java:344)
	at com.jme3.export.binary.BinaryImporter.load(BinaryImporter.java:242)
	at com.jme3.export.binary.BinaryImporter.load(BinaryImporter.java:125)
	at com.jme3.export.binary.BinaryImporter.load(BinaryImporter.java:109)
	at com.jme3.asset.DesktopAssetManager.loadAsset(DesktopAssetManager.java:288)
	at com.jme3.asset.DesktopAssetManager.loadModel(DesktopAssetManager.java:374)
	at com.jme3.gde.core.assets.SpatialAssetDataObject.loadAsset(SpatialAssetDataObject.java:94)
	at com.jme3.gde.scenecomposer.OpenSceneComposer$1.run(OpenSceneComposer.java:38)
	at java.lang.Thread.run(Thread.java:745)
SEVERE [org.openide.util.Exceptions]
java.lang.NullPointerException
	at com.jme3.scene.Spatial.clone(Spatial.java:1188)
	at com.jme3.scene.Node.clone(Node.java:564)
	at com.jme3.scene.Node.clone(Node.java:60)
	at com.jme3.scene.Spatial.clone(Spatial.java:1173)
	at com.jme3.scene.Node.clone(Node.java:564)
	at com.jme3.scene.Node.clone(Node.java:60)
	at com.jme3.scene.Spatial.clone(Spatial.java:1218)
	at com.jme3.scene.Spatial.clone(Spatial.java:66)
	at com.jme3.asset.CloneableAssetProcessor.createClone(CloneableAssetProcessor.java:48)
	at com.jme3.asset.DesktopAssetManager.loadAsset(DesktopAssetManager.java:327)
	at com.jme3.asset.DesktopAssetManager.loadModel(DesktopAssetManager.java:374)
[catch] at com.jme3.gde.core.assets.SpatialAssetDataObject.loadAsset(SpatialAssetDataObject.java:94)
	at com.jme3.gde.scenecomposer.OpenSceneComposer$1.run(OpenSceneComposer.java:38)
	at java.lang.Thread.run(Thread.java:745)
```