Console: Provide icon unknown nodes in the tree

When the console tries to show a node at a path that does not exist it may show up in the tree as "unknown". If the edit panel on the right tries to render this node as well, an NPE is thrown. That should be prevented.

Reproduction path:
1. export a node from the repository as xml (e.g. /content/assets)
2. delete the node and save
3. import the same node again using the xml (don't save!)
4. select the new node in the tree
5. click "Reset" top right and give OK in the dialog

Result: in the tree the node still appears with name "unknown", but the console seems stuck, refresh leads to screen "Server Error". Stacktrace in the logs:

[INFO] [talledLocalContainer] Caused by: java.lang.NullPointerException
[INFO] [talledLocalContainer] at org.hippoecm.frontend.plugins.console.icons.JcrNodeIcon.isNodeType(JcrNodeIcon.java:272)
[INFO] [talledLocalContainer] at org.hippoecm.frontend.plugins.console.icons.JcrNodeIcon.getIconTypeCssClass(JcrNodeIcon.java:198)
[INFO] [talledLocalContainer] at org.hippoecm.frontend.plugins.console.icons.JcrNodeIcon.getIconCssClass(JcrNodeIcon.java:185)
etc.