[BUG][typescript-fetch] Default case for oneOf serialization method returning undefined variable.

[btpnlsl]

Closes #21668

@ThorinEk reported a problem in the discussion of Issue 21587 that relates to the default case in the switch statement generated for a oneOf model when the model has a discriminator.

export function ExecuteCommandPostRequestToJSONTyped(value?: ExecuteCommandPostRequest | null, ignoreDiscriminator: boolean = false): any {
    if (value == null) {
        return value;
    }
    switch (value['command']) {
        ...  // removed
        default:
            // this should be return value since the variable json is undefined.
            return json;
    }
}

It looks like this bug was introduced in the PR for #20983

I did not implement a unit test for this bug because the typescript unit tests are asserting on the failure to match the presence of a string in a specific file. I've looked through the codebase and it looks like there has been some work by @DavidGrath to use antlr to build a Kotlin parser for testing for proper syntax. This can probably be done for Typescript & Javascript too.

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Typescript: @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)
Prior work: PR #20983 - @aeneasr

[macjohnny]

thanks for the fix!