Fix DreamBackendTest

In the test, getStringArray is currently returning null which is causing
a null pointer exception. This only happens in testing when we mock
Resources, and it cannot return null in prod.

Test: atest SettingsLibRoboTests:DreamBackendTest
Change-Id: I8d7e21e26d3d786fce977c530f7537c549a1b678

packages/SettingsLib/tests/robotests/src/com/android/settingslib/dream/DreamBackendTest.java

@@ -57,6 +57,9 @@ public final class DreamBackendTest {
         when(res.getIntArray(
                 com.android.internal.R.array.config_dreamComplicationsEnabledByDefault)).thenReturn(
                 DEFAULT_DREAM_COMPLICATIONS);
+        when(res.getStringArray(
+                com.android.internal.R.array.config_disabledDreamComponents)).thenReturn(
+                new String[]{});
         mBackend = new DreamBackend(mContext);
     }