Prevents empty string entitlement plmn case.

When reading the entitlement plmn list from db after the boot time, if it is an empty string, this value is also considered as an empty list and the entitlement PLMN list cache is not updated.

Bug: 329536793
Bug: 330442524
Test: atest SatelliteControllerTest
Test: atest SubscriptionManagerServiceTest
Test: manual
1. When data or wifi is off and after reading from the db and getting the empty string list, verify no update the entitlementPlmnList cache.
2. After data or wifi is on and EntitlementStatus as false and plmnlist is empty, verify whether entitlementPlmnList is not used in MergedPlmnList.
3. After clear cache, verify whether entitlementPlmnList is not used in MergedPlmnList.
4. After data or wifi is on and EntitlementStatus as true and plmnlist is available [31026, 302820] , verify whether entitlementPlmnList is used in MergedPlmnList.
5. After clear cache, verify whether entitlementPlmnList is used in MergedPlmnList.
6. After data or wifi is on and EntitlementStatus as true and plmnlist is available but includes empty string plmn [31026, ], verify whether filteredEntitlementPlmnList [31026] is used in MergedPlmnList.
7. After clear cache, verify whether entitlementPlmnList [31026] is used in MergedPlmnList.
8. After data or wifi is on and EntitlementStatus as true and plmnlist is all empty string plmn list [, ], verify whether entitlementPlmnList is not used in MergedPlmnList.
9. After clear cache, verify whether entitlementPlmnList is not used in MergedPlmnList.
10. After data or wifi is on and EntitlementStatus as true and plmnlist is available but includes empty string plmn [, 302820] , verify whether filteredEntitlementPlmnList [302820] is used in MergedPlmnList.
11. After clear cache, verify whether entitlementPlmnList [302820] is used in MergedPlmnList.

Change-Id: I3252f2607838c4cbf068e5e33b312a02f65ba104