External Exporters bug fix

[qqmyers]

What this PR does / why we need it: When there is no weight variable (which is tested in an IT test but not a unit test), the code give an NPE. This PR checks for a null response in getWeghtvariable() to avoid that.

This is a quick follow-up to #9175 which was merged before the IT test failure was noticed.

Which issue(s) this PR closes:

Closes #

Special notes for your reviewer:

Suggestions on how to test this: Let the IT tests run/succeed.

Does this PR introduce a user interface change? If mockups are available, please link/include them here:

Is there a release notes update needed for this change?:

Additional documentation:

[pdurbin]

I'm approving this but a couple thoughts:

• Are we worried about more NPEs from this part of the code?
• Should we switch to NullSafeJsonBuilder? Nevermind. It already is. 😞

[pdurbin]

I'm just mentioning which test is failing and how:

Expected status code <200> doesn't match actual status code <at edu.harvard.iq.dataverse.api.DatasetsIT.testCuratePublishedDatasetVersionCommand(DatasetsIT.java:2787)

From here, for example: https://jenkins.dataverse.org/job/IQSS-Dataverse-Develop-PR/job/PR-9493/4/testReport/edu.harvard.iq.dataverse.api/DatasetsIT/testCuratePublishedDatasetVersionCommand/