Log when returning empty system routes list

Bug: 349550462
Change-Id: I41891ddcb60aeef143be7ae9d47ef36579d9c383
Test: atest CtsMediaBetterTogetherTestCases
Flag: EXEMPT Logging only.
diff --git a/services/core/java/com/android/server/media/MediaRouter2ServiceImpl.java b/services/core/java/com/android/server/media/MediaRouter2ServiceImpl.java
index ba7d3b8..d9f3622 100644
--- a/services/core/java/com/android/server/media/MediaRouter2ServiceImpl.java
+++ b/services/core/java/com/android/server/media/MediaRouter2ServiceImpl.java
@@ -251,6 +251,10 @@
                         systemRoutes = providerInfo.getRoutes();
                     } else {
                         systemRoutes = Collections.emptyList();
+                        Slog.e(
+                                TAG,
+                                "Returning empty system routes list because "
+                                    + "system provider has null providerInfo.");
                     }
                 } else {
                     systemRoutes = new ArrayList<>();