Fix broken test in WifiEnterpriseRestrictionUtils

- The original design would result in NPE if the user did not have any
  restrictions.

- Refine to use UserManager#hasUserRestriction instead of
  UserManager#getUserRestrictions.

Bug: 229350205
Test: manual test
make RunSettingsRoboTests ROBOTEST_FILTER=WifiSettingsTest
make RunSettingsLibRoboTests \
  ROBOTEST_FILTER=WifiEnterpriseRestrictionUtilsTest

Change-Id: Iec60e3a1c2778fc89596b3c4d28a34604adfdfb8