[Wi-Fi] Fix unit test ssidPref_isSubscription_show

Only setSummary for mSsidPref when the SSID is not null

Bug: 152375927
Test: make RunSettingsRoboTests ROBOTEST_FILTER=WifiDetailPreferenceController2Test
Change-Id: Ia39f01e6bdb7b1b531a6b07610e061a98140dda4

tests/robotests/src/com/android/settings/wifi/details2/WifiDetailPreferenceController2Test.java

@@ -723,6 +723,7 @@ public class WifiDetailPreferenceController2Test {
     public void ssidPref_isSubscription_show() {
         setUpForConnectedNetwork();
         when(mMockWifiEntry.isSubscription()).thenReturn(true);
+        when(mMockWifiEntry.getSsid()).thenReturn(SSID);
 
         displayAndResume();