[typescript-fetch] Fix response type to be Response or undefined

Created by: GlennOlsson

Current generated code produces a response variable set to undefined, and TS does not like you changing the type of the variable later. Therefore, set the type of the variable to be Response or undefined

Solves OpenAPITools/openapi-generator#12007

Very small change, simply fixing the type of the response variable so Typescript doesn't complain

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