Fix an NPE when a user is added and then removed quickly

If a user is added and then removed quickly, the user id in
user-added intent received by the VPN to retrieve UserInfo might
return null. Using this invalid UserInfo for checks can cause an
NPE. Therefore, add a null check for the retrieved UserInfo
before using it.

Bug: 382602272
Test: atest FrameworksVpnTests
Change-Id: I14091d6728bf7e8cfaefdff7d1687a391bd0663f