array_join adds an extra delimeter if the last element is null

[kevinwilfong]

Description

array_join adds an extra trailing delimeter to the result string if the last element in the array is null

Motivation and Context

this seems to be clearly incorrect, if the null replacement argument isn't set nulls are otherwise ignored by array_join

Impact

this will change the results of array_join for cases where the last element is null

Test Plan

added a testcase where the null replacement argument is not set and the last element is null, this was previously untested

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Release Notes

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== RELEASE NOTES ==

General Changes
* Fix array_join to not add a trailing delimeter when the last element in the array is NULL. :pr:`22652`

[mbasmanova]

CC: @tdcmeehan @aditi-pandit @steveburnett

[steveburnett]

Nit: suggest adding the PR to the release note entry.

== RELEASE NOTES ==

General Changes
* Fix array_join to not add a trailing delimeter when the last element in the array is NULL. :pr:`22652`

[tdcmeehan]

Great fix, just some nits. Thanks!

[kevinwilfong]

Thanks for the review!