Relax the warning of unavailable tokens in `gen_defaults` when a default value is provided (#140872)

Fixes some false positives and false negatives from the `gen_defaults` tool regarding unavailable tokens in the Material spec.

Fixes #140871 

# Before

![image](https://github.com/flutter/flutter/assets/22084723/eb6c43ee-d919-4203-80ee-e36869e5bbcf)

# After

![image](https://github.com/flutter/flutter/assets/22084723/67093dcb-1ab0-439f-9338-a6f364d2a9e1)

These new issues that are logged are being fixed in https://github.com/flutter/flutter/pull/140573

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