[Settings] Change the way in MobileNetworkListFragment for getting the subscription info from room db part3

Bug: 254405469
Test: atest NetworkProviderDownloadedSimListControllerTest, atest NetworkProviderSimListControllerTest
Change-Id: Iec6e17688b260d2fef464a711f92e788fe76fbe7

src/com/android/settings/network/SubscriptionUtil.java

@@ -610,7 +610,7 @@ public class SubscriptionUtil {
         boolean isDefaultData = subId == getDefaultDataSubscriptionId();
 
         if (!isDefaultData && !isDefaultCall && !isDefaultSms) {
-            return null;
+            return "";
         }
 
         final StringBuilder defaultConfig = new StringBuilder();