chg: ensure actor, target and support ticket are non-null

chg: ensure actor, target and support ticket are non-null

This patch is a defense-in-depth in cases of internal inconsistencies:
If e.g. somehow the session used to authenticate a request gets corrupted or
through a race condition a user gets deleted in the DB between validation
of the password and passing on that user to the actual session login this
will provide a safe-guard. This also centralises the check for acceptable
AuthorisationContexts in the class itself.

Result of this patch is any call to AuthorisationContext.getActor() will
return a non-null User object, as AuthorisationContexts with an null actor,
target or support ticket are rejected as invalid.

Change-Id: I78009fe3385820cd46a31a74c4c68f1cdaa65628