fix: dont warn when no remote tags exist

[teto]

My logs were full of scary warnings "Could not parse latest tag from
..".

turns out if a remote repo has no tags, we were trying to decode an
empty string parser.parse_git_latest_semver_tag(sc.stdout or "").
I special cased the empty-string (ie, no remote tags) so it appears as a debug level.

The test was not necessary but I added it while debugging and it doesn't hurt to have it.

I still have illegitimate failures I intend to fix as follow up

INFO | 2026-07-27 19:04:31.403 | ...m/start/rocks.nvim/lua/rocks/operations/helpers/init.lua:164 | Installed: menu -> scm
INFO | 2026-07-27 19:04:31.403 | /home/teto/plugins/rocks-git.nvim/lua/rocks-git/git.lua:38 | { "git", "ls-remote", "--tags", "https://github.com/elanmed/fzf-lua-frecency.nvim.git" }
WARN | 2026-07-27 19:04:31.817 | /home/teto/plugins/rocks-git.nvim/lua/rocks-git/git.lua:262 | Usign url https://github.com/elanmed/fzf-lua-frecency.nvim.git
DEBUG | 2026-07-27 19:04:31.818 | /home/teto/plugins/rocks-git.nvim/lua/rocks-git/git.lua:267 | No tag for https://github.com/elanmed/fzf-lua-frecency.nvim.git
INFO | 2026-07-27 19:04:31.821 | /home/teto/plugins/rocks-git.nvim/lua/rocks-git/git.lua:38 | { "git", "checkout", "ref: refs/remotes/up/master", "--force", "--recurse-submodules" }
ERROR | 2026-07-27 19:04:31.849 | /home/teto/plugins/rocks-git.nvim/lua/rocks-git/git.lua:180 | { "Could not checkout HEAD branch", "ref: refs/remotes/up/master", {
    code = 1,
    signal = 0,
    stderr = "erreur : le spécificateur de chemin 'ref: refs/remotes/up/master' ne correspond à aucun fichier connu de git\n",
    stdout = ""
  } }