Allow shutdown() to NOOP if we were never init()ed.

src/java/com/samskivert/depot/PersistenceContext.java

@@ -184,8 +184,10 @@ public class PersistenceContext
         } catch (Throwable t) {
             log.warning("Failure shutting down Depot cache.", t);
         }
+        if (_conprov != null) {
             _conprov.shutdown();
         }
+    }
 
     /**
      * Returns a snapshot of our current runtime statistics.