useSingleRequestParameter should mark parameter optional if all properties are optional

[davidgamero]

This PR uses a default value to requestParameters (= {}) if all the parameters are optional.

Fixes #11112

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[amakhrov]

Thanks for your PR. Looks great (and I even feel other TS generators would benefit from a similar change, too).

[davidgamero]

Thank you! Just need a maintainer approval to run workflow next