Add "undefined" as an option for generator fuel

[ovasios]

Merge request type

• [ ] New feature
• [ ] Resolves bug
• [ ] Documentation
• [ ] Other

Relates to

• [ ] OPFLOW
• [ ] SOPFLOW
• [ ] SCOPFLOW
• [ ] TCOPFLOW
• [ ] CMake build system
• [ ] Spack configuration
• [ ] Manual
• [ ] Web docs
• [ ] Other

This MR updates

• [ ] Header files
• [ ] Source code
• [ ] CMake build system
• [ ] Spack configuration
• [ ] Web docs
• [ ] Manual
• [ ] Other

Summary

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Linked Issue(s)

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• Closes #issue
• Resolves #issue

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• issue

• Partially addresses #issue

[abhyshr]

Looks good.