Nullable annotation fixes for consistency

[tillig]

Fixes #40.

Issue #40 indicated that the ITenantIdentificationStrategy.TryIdentifyTenant was allowing for a null return value if the method returned true.

While I think this is a super edge case, technically speaking we do have the concept of a "default tenant" in the system that is indicated by a null tenant ID. Thus, it is technically possible someone may want to say, "I looked, and I intentionally have determined that we should use the default tenant ID, thus I am intentionally setting the return value to null."

I found that we weren't correctly handling all nullable annotations in MultitenantContainer, so that has been updated. I've also added some documentation to explain the potential intentional null; and some unit tests to illustrate more of the notion of intentional identification-as-null.

The other option here would be to make a change to the TryIdentifyTenant method definition such that it's not allowed to return null if the tenant is identified. I think it's possible this is a breaking change given that sort of behavior is currently allowed and we've definitely seen people doing every possible edge case out there, even if it's not the 80% case. If we did that, it may be that we need to have a public/singleton for the default tenant ID such that an intentional identification as the default tenant will result in a non-null tenant ID. Not a huge lift, but not just a nullable annotation change, either.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93.57%. Comparing base (1a7a9eb) to head (9f33d44).

Additional details and impacted files

@@           Coverage Diff            @@
##           develop      #41   +/-   ##
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  Coverage    93.57%   93.57%           
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  Files            5        5           
  Lines          109      109           
  Branches        28       28           
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  Hits           102      102           
  Misses           3        3           
  Partials         4        4           

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[alistairjevans]

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