[ONOS-7406] Fix NPE when annotation is null

Change-Id: I1e51e6921c938d5f3e33c64338c5d1f645b45117
diff --git a/cli/src/main/java/org/onosproject/cli/AbstractShellCommand.java b/cli/src/main/java/org/onosproject/cli/AbstractShellCommand.java
index f83c5fa..09e08fa 100644
--- a/cli/src/main/java/org/onosproject/cli/AbstractShellCommand.java
+++ b/cli/src/main/java/org/onosproject/cli/AbstractShellCommand.java
@@ -28,6 +28,7 @@
 
 import com.fasterxml.jackson.databind.ObjectMapper;
 import com.fasterxml.jackson.databind.node.ObjectNode;
+import org.onosproject.net.DefaultAnnotations;
 
 import java.util.Set;
 import java.util.TreeSet;
@@ -90,6 +91,9 @@
      * @return string image with ", k1=v1, k2=v2, ..." pairs
      */
     public static String annotations(Annotations annotations) {
+        if (annotations == null) {
+            annotations = DefaultAnnotations.EMPTY;
+        }
         StringBuilder sb = new StringBuilder();
         Set<String> keys = new TreeSet<>(annotations.keys());
         for (String key : keys) {