Allow for null revision resolver when using repo embeddings job's empty revision string

[gl-srgr]

This PR fixed an issue with job scheduling/execution in the backend. The interim fix of this PR relies on allowing for empty string revision values written with a repo embedding job when the repo is empty or some other issue occurs when fetching the repo's default branch (e.g. empty repo).

Empty revision value causes issues with graphql query repoEmbeddingJobs when querying for Revision as our site admin jobs panel does.

Panic occurred: runtime error: slice bounds out of range [:7] with length 0

This PR updates the embedding job resolver to consider empty revision string acceptable and not call constructor for git commit resolver. Also site admin jobs list will now show repo name if repo is non-null and revision is null.

Test plan

sg manual test

[sourcegraph-bot]

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[github-actions]

The backport to 5.1 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 working tree
git worktree add .worktrees/backport-5.1 5.1
# Navigate to the new working tree
cd .worktrees/backport-5.1
# Create a new branch
git switch --create backport-54879-to-5.1
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick -x --mainline 1 1dd6db4e2d507be7a8c41cec3882290100383e0c
# Push it to GitHub
git push --set-upstream origin backport-54879-to-5.1
# Go back to the original working tree
cd ../..
# Delete the working tree
git worktree remove .worktrees/backport-5.1

Then, create a pull request where the base branch is 5.1 and the compare/head branch is backport-54879-to-5.1.