Fix incorrect result of arrayElement / map[] on empty value

[Algunenano]

Changelog category (leave one):

• Bug Fix (user-visible misbehavior in an official stable release)

Changelog entry (a user-readable short description of the changes that goes to CHANGELOG.md):

• Fix incorrect result of arrayElement / map[] on empty value

Documentation entry for user-facing changes

Introduced in #55957. The values stored are copied using memcpySmallAllowReadWriteOverflow15 which requires the source to have at least 15 extra bytes at the end which might be copied into the target. zero_buf did not respect that.

Closes #59402

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