[rust-server] Restore support for nullable

Created by: bjgill

Accounts for changes in https://github.com/OpenAPITools/openapi-generator/pull/930. Adds a nullable field to the sample to reduce the risk of this regressing again in future.

PR checklist

• Read the contribution guidelines.
• Ran the shell script under ./bin/ to update Petstore sample so that CIs can verify the change. (For instance, only need to run ./bin/{LANG}-petstore.sh and ./bin/security/{LANG}-petstore.sh if updating the {LANG} (e.g. php, ruby, python, etc) code generator or {LANG} client's mustache templates). Windows batch files can be found in .\bin\windows\.
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Description of the PR

Accounts for changes in #930. Adds a nullable field to the sample to reduce the risk of this regressing in future.

This PR basically renames x-nullable to isNullable in the mustache template.