Fix NPE when clean up SingleSwitchFibInstaller

interfaces can be null if no interface config is provided

Change-Id: I67f94f78ef1fb4536e5bbc940be6216e2699f2fe
diff --git a/apps/routing/src/main/java/org/onosproject/routing/impl/SingleSwitchFibInstaller.java b/apps/routing/src/main/java/org/onosproject/routing/impl/SingleSwitchFibInstaller.java
index a295a14..dcdba8e 100644
--- a/apps/routing/src/main/java/org/onosproject/routing/impl/SingleSwitchFibInstaller.java
+++ b/apps/routing/src/main/java/org/onosproject/routing/impl/SingleSwitchFibInstaller.java
@@ -282,7 +282,7 @@
 
     private Set<Interface> getInterfaces() {
         Set<Interface> intfs;
-        if (interfaces.isEmpty()) {
+        if (interfaces == null || interfaces.isEmpty()) {
             intfs = interfaceService.getInterfaces();
         } else {
             // TODO need to fix by making interface names globally unique