GCC complains about empty format strings, and also clang already
required special-handling for this case.
We could either drop it, since statically empty strings are not that
useful anyway, but for completeness I fix it via "if constexpr" instead.

R=tebbi@chromium.org

Bug: chromium:1323177
Change-Id: I4d59e1b361afd1edcd552e8a9ce395759646e67f
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3644433Reviewed-by: Tobias Tebbi <tebbi@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/main@{#80540}