[Models] Enable nullable (2 of 2)

[iamcarbon]

This PR enables nullable for the Models project and updates a few properties in the Metadata namespace to match their nullability with specs.

[codecov-commenter]

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Codecov Report

Attention: Patch coverage is 56.25000% with 7 lines in your changes missing coverage. Please review.

Project coverage is 76.04%. Comparing base (b7f1735) to head (5dd1211).

Files with missing lines	Patch %	Lines
Src/Fido2.Models/Metadata/BiometricStatusReport.cs	0.00%	5 Missing ⚠️
Src/Fido2.Models/CredentialCreateOptions.cs	0.00%	2 Missing ⚠️

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Additional details and impacted files

@@           Coverage Diff           @@
##           master     #585   +/-   ##
=======================================
  Coverage   76.03%   76.04%           
=======================================
  Files         101      101           
  Lines        2600     2601    +1     
  Branches      432      432           
=======================================
+ Hits         1977     1978    +1     
  Misses        513      513           
  Partials      110      110           

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[abergs]

LGTM, thank you!