[RestTemplate] Update ApiClient.mustache to fix openApiNullable

[quarantam]

Fixing #7209

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[wing328]

cc @bbdouglas (2017/07) @sreeshas (2017/08) @jfiala (2017/08) @lukoyanov (2017/09) @cbornet (2017/09) @jeff9finger (2018/01) @karismann (2019/03) @Zomzog (2019/04) @lwlee2608 (2019/10) @nmuesch (2021/01)

[wing328]

Tested locally and the result is good. Will update the samples in another commit after merging this PR into the master.