[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
1 file changed