[normalizer] Better handling of schema with just description (any type)

[wing328]

to close #20459

fyi @OpenAPITools/generator-core-team

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[wing328]

FYI. This fix the incorrect null (as any type is incorrectly treated as null in openapi normalizer)

@TiFu (2017/07) @taxpon (2017/07) @sebastianhaas (2017/07) @kenisteward (2017/07) @Vrolijkx (2017/09) @macjohnny (2018/01) @topce (2018/10) @akehir (2019/07) @petejohansonxo (2019/11) @amakhrov (2020/02) @davidgamero (2022/03) @mkusaka (2022/04) @joscha (2024/10)