fix(customeradministration): TargetId should be nullable.

[kporras07]

Otherwise it gets sent in the query and we get no results even if there are expected results

[codecov-commenter]

Codecov Report

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

Project coverage is 38.77%. Comparing base (3ff4ee2) to head (8b790e9). Report is 42 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #1173 +/- ## ========================================== - Coverage 38.84% 38.77% -0.07% ========================================== Files 86 93 +7 Lines 5612 4722 -890 ========================================== - Hits 2180 1831 -349 + Misses 3266 2716 -550 - Partials 166 175 +9 ``` | [Flag](https://app.codecov.io/gh/newrelic/newrelic-client-go/pull/1173/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=newrelic) | Coverage Δ | | |---|---|---| | [unit](https://app.codecov.io/gh/newrelic/newrelic-client-go/pull/1173/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=newrelic) | `38.77% <ø> (-0.07%)` | :arrow_down: | 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=newrelic#carryforward-flags-in-the-pull-request-comment) to find out more.

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[pranav-new-relic]

@kporras07 👍 will quickly perform one final check to make sure no unexpected tests are breaking, and will go ahead and merge this.