Update nullable annotations to correct handling of tenant ID

[tillig]

Two scenarios had incorrect nullable annotations:

1. When identifying a tenant (TryIdentifyTenant), if the tenant was not successfully identified, the output tenant ID should be allowed to be null.
2. When configuring a tenant lifetime scope, ConfigureTenant should allow a null tenant ID to indicate the "default tenant."

As part of this I enabled nullable annotations on the unit test projects to ensure things were lining up and allow us to see the nullable warnings when violations in test fixtures occur.

Includes a semver update from 8.0.1 => 8.0.2 for this fix. (My commit message has a typo. 😦 )

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93.57%. Comparing base (39f9710) to head (3ea67e1).

Additional details and impacted files

@@           Coverage Diff            @@
##           develop      #39   +/-   ##
========================================
  Coverage    93.57%   93.57%           
========================================
  Files            5        5           
  Lines          109      109           
  Branches        28       28           
========================================
  Hits           102      102           
  Misses           3        3           
  Partials         4        4           

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[alistairjevans]

👍