CxxUtils+AthenaKernel: Fix problem with trimming last element.

Scott Snyderrequested to merge
ssnyder/athena:trimlast.CxxUtils-20230424 into 23.0

We forgot to get m_last to nullptr if the last element gets removed by trim(), which was leading to use-after-free errors. Fixed. Restore the change addressing ATLASRECTS-7421.

See also ATLASDQ-1073.

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