[typescript-fetch] Use == to check property is null or undefined

[kota65535]

Fix: #17970

The root cause of the issue is that Typescript compiler does not understand that the exists() function ensures the property is not null or undefined. This PR replaces all exists() with == operator, which is more efficient way to do the same thing.
I've also refactored some verbose code like if (json === undefined || json === null) to if (json == null).

To committee: @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)

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[wing328]

thanks for the PR. let's give it a try