[typescript-axios] Add temination condition for flattening url parameters

Created by: teddy-s-song

Resolved Type Error - TypeError: Cannot convert undefined or null to object With typescript-axios template, function setFlattenedQueryParams which stringifies url search parameters raises TypeError when value of parameter is null (like cursor: null) or undefined (like obj: { key: undefined } in recursive execution).

It seems to be same issue with #5174 issue and #5197 PR for typescript-angular.

@mkusaka

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