fix(ec2): Ensure metadata exists before configuring PBR

[TheRealFalcon]

Draft as currently untested.

Proposed Commit Message

fix(ec2): Ensure metadata exists before configuring PBR

Fixes GH-5283

Additional Context

https://github.com/canonical/cloud-init/issues/5283

Test Steps

Currently untested

Merge type

• [x] Squash merge using "Proposed Commit Message"
• [ ] Rebase and merge unique commits. Requires commit messages per-commit each referencing the pull request number (#<PR_NUM>)