[typescript] Skip missing fields during deserialization

[bodograumann]

Currently the deserializer fills all fields which are missing in the payload with the value undefined. Usually the difference between a missing field and a field which is set to undefined is not important. In some cases the behaviour is different however. For example when using Object.keys, the in operator or a strict recursive comparison like lodash’s isEqual.

This PR changes the behaviour, so that the missing fields are skipped and the deserialized objects do not have them set to undefined anymore.
I feel this is the correct approach, because this is the most strict meaning for optional properties in typescript. (Even though the flag that checks strictly is not enabled by default, for backward compatibility.)

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CC @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)

[macjohnny]

LGTM