Fix error adding log entry with no body

[JamesNK]

Description

Fixes #6703

Checklist

• Is this feature complete?
  • Yes. Ready to ship.
  • No. Follow-up changes expected.
• Are you including unit tests for the changes and scenario tests if relevant?
  • Yes
  • No
• Did you add public API?
  • Yes
    • If yes, did you have an API Review for it?
      • Yes
      • No
    • Did you add <remarks /> and <code /> elements on your triple slash comments?
      • Yes
      • No
  • No
• Does the change make any security assumptions or guarantees?
  • Yes
    • If yes, have you done a threat model and had a security review?
      • Yes
      • No
  • No
• Does the change require an update in our Aspire docs?
  • Yes
    • Is this introducing a breaking change?
      • Yes
        • Link to aspire-docs issue (please use this breaking-change template):
      • No
        • Link to aspire-docs issue (please use this doc-idea template):
  • No

Microsoft Reviewers: Open in CodeFlow

[JamesNK]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[drewnoakes]

Wish we had a protobuf generator with null annotations.

[JamesNK]

/backport to 9.0

[github-actions]

Started backporting to 9.0: https://github.com/dotnet/aspire/actions/runs/11965737380

[github-actions]

@JamesNK an error occurred while backporting to 9.0, please check the run log for details!

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

[JamesNK]

/backport to release/9.0

[github-actions]

Started backporting to release/9.0: https://github.com/dotnet/aspire/actions/runs/11965747913