[typescript][fetch] Fix null typing errors

[allejo]

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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

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

After PR

exportfunctionModelToJSON(value?: Model|null): any{if(value===undefined){returnundefined;}if(value===null){returnnull;}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)

[macjohnny]

LGTM

[wing328]

@allejo thanks for the PR, which has been included in the v4.1.3 release: https://twitter.com/oas_generator/status/1180123829626003456