handle null memberuid, refactor

[simonLeary42]

• fixed null memberuid edge case in admin check, UnityGroup, UnityOrg
• renamed UnityGroup::GetLDAPPiGroup() to GetLDAPEntry. Renamed local variables from pi_group to entry. Inside the PI group class, calling a variable pi_group means nothing.
• renamed UnityOrg::GetLDAPOrgGroup() to GetLDAPEntry. Renamed local variables from org_group to entry. Inside the org group class, calling a variable org_group means nothing.
• renamed UnityOrg::isInOrg to userExists and changed input data type to match UnityGroup::userExists
• in both UnityGroup and UnityOrg:
  • move logic from get{Org,Group}Members to get{Org,Group}MemberUIDs, handle null memberuid edge case and make available for userExists
• added type annotations

[simonLeary42]

fixed by hakasapl/phpopenldaper#6