[typescript][fetch] Fix null typing errors

Created by: allejo

PR checklist

• Read the contribution guidelines.
• Ran the shell script under ./bin/ to update Petstore sample so that CIs can verify the change. (For instance, only need to run ./bin/{LANG}-petstore.sh, ./bin/openapi3/{LANG}-petstore.sh if updating the {LANG} (e.g. php, ruby, python, etc) code generator or {LANG} client's mustache templates). Windows batch files can be found in .\bin\windows\. If contributing template-only or documentation-only changes which will change sample output, be sure to build the project first.
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Description of the PR

The current typescript-fetch template generates some invalid Typescript where the typing for the function is either undefined or the model. However, in the other generated code it can also be null and the function itself is generating a null check.

Additionally, for each nullable entity in the return statement, checking against == null will essentially check for both undefined and null since in JS, null == undefined is true.

Current

export function ModelToJSON(value?: Model): any {
    if (value === undefined) {
        return undefined;
    }
    if (value === null) {
        return null;
    }
    return {
        
        'primitive_value': value.primitiveValue,
        'nullable_date': value.nullableDate === undefined ? undefined : value.nullableDate.toISOString(),
        'regular_date': value.regularDate.toISOString(),
    }
}

After PR

export function ModelToJSON(value?: Model | null): any {
    if (value === undefined) {
        return undefined;
    }
    if (value === null) {
        return null;
    }
    return {
        
        'primitive_value': value.primitiveValue,
        'nullable_date': value.nullableDate == null ? undefined : value.nullableDate.toISOString(),
        'regular_date': value.regularDate.toISOString(),
    }
}

Additionally, the indentation of a generated comment is wrong, so that fix is included in here as well.

I don't currently have a Java dev environment setup on my computer but I've tested these template changes through the -t flag and would appreciate any early feedback while I get Java set up to check that last box.

@TiFu (2017/07) @taxpon (2017/07) @sebastianhaas (2017/07) @kenisteward (2017/07) @Vrolijkx (2017/09) @macjohnny (2018/01) @nicokoenig (2018/09) @topce (2018/10) @akehir (2019/07)