Avoid zero-length memory copies with null array or string data

[dand-oss]

Skip memcpy/memmove when the relevant element count is zero in Array copy/insert/growth operations and when AbstractString is constructed from an empty buffer. Empty dynamically allocated arrays can have null storage, which UBSan diagnoses even when the memory-operation length is zero. Preserve count updates and nonempty operations.

This is a fresh proposal related to the closed #8848. It also covers empty-capacity growth and the length/data AbstractString constructor. The adaptation to current master retains the newer static_cast<void*> destinations and allocator signatures.

Validation on Linux x86-64 with Clang 21 / C++20 / -fsanitize=undefined -fno-sanitize-recover=all:

• A focused harness instantiates the actual production Array header with a malloc-backed test storage policy, independently of Firebird's global memory-pool startup.
• Unpatched master fails on empty copy construction, empty-array insertion, null-pointer/zero-count insertion, growing an empty array, and initial growth in a nonempty-operation sequence.
• All five cases pass with the patch, including checks of resulting counts/content after insertion and assignment.
• git diff --check passes.

The isolated harness does not cover Firebird's memory-pool lifecycle. The AbstractString runtime case and a complete patched server build have not yet been completed locally.