fix(nsec3): fixes unset nsec3param not working

[SurajDadral]

Description

$powerdns->zone($domain)->unsetNsec3param()

This wasn't working because {"nsec3param": null} was passed in API.
Now, modified to pass {"nsec3param": ""} in API to unset nsec3param.

How has this been tested?

php: 8.2
powerdns version: 4.9.1-1pdns.buster

All phpunit test cases passed: ./vendor/bin/phpunit

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[trizz]

Hi @SurajDadral

Thanks for the fix! I'll merge and release it.