fix(project): skip defaultBranch late-init for empty repo

[henrysachs]

Description of your changes

Fixes #444

I have:

• Read and followed Crossplane's contribution process.
• Run make reviewable test to ensure this PR is ready for review. Ran go build ./..., go test ./pkg/... ./apis/..., go generate for apis and pkg (no diff), and golangci-lint on touched packages.

Why

A Project without a repository goes Synced: False on its first update. GitLab reports default_branch: main and empty_repo: true for an empty project. Observe late-initialized defaultBranch from that value. Every later update sent default_branch=main back, and GitLab rejects it with Could not change HEAD: branch 'main' does not exist.

What changed

• lateInitialize skips defaultBranch while empty_repo is true. It late-inits as before once the repo has a commit.
• An explicitly set defaultBranch is kept on empty repos, so GitLab's error still surfaces instead of being dropped.
• New table test TestLateInitializeDefaultBranch for empty/non-empty repo with unset/explicit spec.
• Cluster-scoped copies regenerated.

Scope

Project controller late-init only. No API or CRD change. Drift check for defaultBranch is unchanged; an unset spec is never compared.

Verification

• API on GitLab 19.4.2-ee: empty project returns default_branch: main, empty_repo: true. PUT default_branch=main returns 400 Could not change HEAD. PUT description=x returns 200.
• Mutation: forcing the late-init unconditionally fails TestLateInitializeDefaultBranch/EmptyRepoUnset.
• Live on kind with a local provider against the same GitLab:
  • Project with only name: Ready, defaultBranch unset. Description change: Synced=True, GitLab shows the new description.
  • After creating README.md on main via API: empty_repo: false, defaultBranch late-inits to main. A further description change stays Synced=True.
  • Project with explicit defaultBranch: main on an empty repo: update fails with GitLab's Could not change HEAD error, as intended.
  • Delete removes the project.

How has this code been tested

See Verification.