Avoid sending nullptr to memcpy in copyStringInArena

[antonio2368]

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):

Avoid sending nullptr to memcpy in copyStringInArena.

Fix #48470

Problem is that ColumnArray for empty arrays creates StringRef(nullptr, 0)

Documentation entry for user-facing changes

• Documentation is written (mandatory for new features)

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