Fix panic in SpanDeduper when traceID is empty slice

[mdisibio]

What this PR does:
SpanDeduper uses the last byte of the trace ID to shard into a submap of seen values. It didn't protect against an empty trace ID, which led to a panic.

Which issue(s) this PR fixes:
Fixes #

Checklist

• Tests updated
• Documentation added
• CHANGELOG.md updated - the order of entries should be [CHANGE], [FEATURE], [ENHANCEMENT], [BUGFIX]

[github-actions]

This PR must be merged before a backport PR will be created.

[github-actions]

Hello @mdisibio!
Backport pull requests need to be either:

• Pull requests which address bugs,
• Urgent fixes which need product approval, in order to get merged,
• Docs changes.

Please, if the current pull request addresses a bug fix, label it with the type/bug label.
If it already has the product approval, please add the product-approved label. For docs changes, please add the type/docs label.
If the pull request modifies CI behaviour, please add the type/ci label.
If none of the above applies, please consider removing the backport label and target the next major/minor release.
Thanks!

[github-actions]

The backport to r147 failed:

The process '/usr/bin/git' failed with exit code 1

To backport manually, run these commands in your terminal:

# Fetch latest updates from GitHub
git fetch
# Create a new branch
git switch --create backport-3668-to-r147 origin/r147
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick -x d8c69d487eaa1f64af291793ef3b667c133342f0

When the conflicts are resolved, stage and commit the changes:

git add . && git cherry-pick --continue

If you have the GitHub CLI installed:

# Push the branch to GitHub:
git push --set-upstream origin backport-3668-to-r147
# Create the PR body template
PR_BODY=$(gh pr view 3668 --json body --template 'Backport d8c69d487eaa1f64af291793ef3b667c133342f0 from #3668{{ "\n\n---\n\n" }}{{ index . "body" }}')
# Create the PR on GitHub
echo "${PR_BODY}" | gh pr create --title "[r147] Fix panic in SpanDeduper when traceID is empty slice" --body-file - --label "type/bug" --label "backport" --base r147 --milestone r147 --web

Or, if you don't have the GitHub CLI installed (we recommend you install it!):

# Push the branch to GitHub:
git push --set-upstream origin backport-3668-to-r147

# Create a pull request where the `base` branch is `r147` and the `compare`/`head` branch is `backport-3668-to-r147`.

# Remove the local backport branch
git switch main
git branch -D backport-3668-to-r147