[C++] Better handling `mem*` api that behave sanely on NULL input

I think we take a systematic approach to this and make wrappers to `mem*` functions that behave sanely on `NULL` input.

This article [1] discusses the issue in detail and points to approaches taken by projects like `boringssl`. [2] There is also a proposal from prolific LLVM contributors for fixing this at the C standard level. [3]

[1] https://davidben.net/2024/01/15/empty-slices.html
[2] https://boringssl.googlesource.com/boringssl/+/17cf2cb1d226b0ba2401304242df7ddd3b6f1ff2%5E%21/
[3] https://docs.google.com/document/d/1guH_HgibKrX7t9JfKGfWX2UCPyZOTLsnRfR6UleD1F8/edit?tab=t.0

_Originally posted by @felipecrv in https://github.com/apache/arrow/issues/44794#issuecomment-2494059369_