Avoid unnecessary empty list instantiation

Change-Id: I128485be4e0d7d3400741855e4519f441cfb34c5

@@ -142,7 +142,7 @@ public class FlowModBuilderVer10 extends FlowModBuilder {
             switch (i.type()) {
             case DROP:
                 log.warn("Saw drop action; assigning drop action");
-                return new LinkedList<>();
+                return Collections.emptyList();
             case L2MODIFICATION:
                 acts.add(buildL2Modification(i));
                 break;

@@ -194,7 +194,7 @@ public class FlowModBuilderVer13 extends FlowModBuilder {
         for (Instruction i : treatments) {
             switch (i.type()) {
                 case DROP:
-                    return new LinkedList<>();
+                    return Collections.emptyList();
                 case L0MODIFICATION:
                     actions.add(buildL0Modification(i));
                     break;

@@ -51,6 +51,7 @@ import org.projectfloodlight.openflow.types.VlanPcp;
 import org.slf4j.Logger;
 
 import java.util.ArrayList;
+import java.util.Collections;
 import java.util.LinkedList;
 import java.util.List;
 import java.util.Optional;
@@ -179,16 +180,16 @@ public final class GroupModBuilder {
     }
 
     private List<OFAction> buildActions(TrafficTreatment treatment) {
-        List<OFAction> actions = new LinkedList<>();
         if (treatment == null) {
-            return actions;
+            return Collections.emptyList();
         }
 
+        List<OFAction> actions = new LinkedList<>();
         for (Instruction i : treatment.allInstructions()) {
             switch (i.type()) {
                 case DROP:
                     log.warn("Saw drop action; assigning drop action");
-                    return new LinkedList<>();
+                    return Collections.emptyList();
                 case L0MODIFICATION:
                     actions.add(buildL0Modification(i));
                     break;