fix: avoid printing duplicate null nodes

[mrlubos]

Closes https://github.com/hey-api/openapi-ts/issues/968

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Latest commit: 11ee53f4f519643ae7e3f0997ce4fe9b6cb050a8

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Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 81.21%. Comparing base (6b4b117) to head (11ee53f). Report is 2 commits behind head on main.

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```diff @@ Coverage Diff @@ ## main #1095 +/- ## ======================================= Coverage 81.21% 81.21% ======================================= Files 102 102 Lines 11503 11506 +3 Branches 1231 1233 +2 ======================================= + Hits 9342 9345 +3 Misses 2158 2158 Partials 3 3 ``` | [Flag](https://app.codecov.io/gh/hey-api/openapi-ts/pull/1095/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=hey-api) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/hey-api/openapi-ts/pull/1095/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=hey-api) | `81.21% <100.00%> (+<0.01%)` | :arrow_up: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=hey-api#carryforward-flags-in-the-pull-request-comment) to find out more.

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commit: 11ee53f