Fix NRE in CompareRequired

[konrad-jamrozik]

Fixes errors like this one:

• https://dev.azure.com/azure-sdk/internal/_build/results?buildId=3975850&view=logs&j=98f87804-8e1f-5655-af02-e80aefa7aa97&t=65457206-7ab6-5966-2a2f-9ae3efa14a03&l=54
• https://github.com/Azure/azure-rest-api-specs/pull/29918/checks?check_run_id=27647800156

Command failed: dotnet \"/mnt/vss/_work/_tasks/AzureApiValidation_5654d05d-82c1-48da-ad8f-161b817f6d41/0.0.111/common/temp/node_modules/.pnpm/@[email protected]/node_modules/@azure/oad/dlls/OpenApiDiff.dll\" -o /tmp/oad-9EHgIA/old-resolved.json -n /tmp/oad-OSxSXz/new-resolved.json
Unhandled exception. System.NullReferenceException: Object reference not set to an instance of an object.
   at AutoRest.Swagger.Model.Schema.CompareRequired(ComparisonContext`1 context, Schema priorSchema) in D:\\a\\_work\\1\\s\\openapi-diff\\src\\modeler\\AutoRest.Swagger\\Model\\Schema.cs:line 214
   at AutoRest.Swagger.Model.Schema.Compare(ComparisonContext`1 context, Schema previous) in D:\\a\\_work\\1\\s\\openapi-diff\\src\\modeler\\AutoRest.Swagger\\Model\\Schema.cs:line 181
   at AutoRest.Swagger.Model.Schema.CompareProperties(ComparisonContext`1 context, Schema priorSchema) in D:\\a\\_work\\1\\s\\openapi-diff\\src\\modeler\\AutoRest.Swagger\\Model\\Schema.cs:line 270
   at AutoRest.Swagger.Model.Schema.Compare(ComparisonContext`1 context, Schema previous)

[konrad-jamrozik]

Deployed with:

• Pull Request 566867: Bump @azure/oad to ~0.10.13