Fix bug that the dream is set to "" when null is passed.

Currently if null is passed, the active dream is set to "" instead of
null. This is problematic because our code doesn't have logic for
handling "" like null.

Test: atest NudgeToSetupDreamCallbackTest
Test: manually flash device and run
Test: atest DreamManagerServiceTests
Bug: 213368707
Change-Id: Ic4922fb61f7ceef9fc48e81f1455872861e34af5

services/core/java/com/android/server/dreams/DreamManagerService.java

@@ -484,14 +484,15 @@ public final class DreamManagerService extends SystemService {
     }
 
     private static String componentsToString(ComponentName[] componentNames) {
+        if (componentNames == null) {
+            return null;
+        }
         StringBuilder names = new StringBuilder();
-        if (componentNames != null) {
-            for (ComponentName componentName : componentNames) {
-                if (names.length() > 0) {
-                    names.append(',');
-                }
-                names.append(componentName.flattenToString());
+        for (ComponentName componentName : componentNames) {
+            if (names.length() > 0) {
+                names.append(',');
             }
+            names.append(componentName.flattenToString());
         }
         return names.toString();
     }