fix: Gracefully handle PR creation when no commits exist

[pratik-mahalle]

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Closes #8167.

When the integration branch is in sync with `main`, `gh pr create` fails with:

\`\`\`
GraphQL: No commits between main and [branch] (createPullRequest)
\`\`\`

This caused the workflows to fail unnecessarily, even though this is a harmless condition.

Changes

Applied the same fix pattern from `refcache-refresh.yml` to:

• `update-spec-integration-branch.yml`
• `update-semconv-integration-branch.yml`

Related

Fixes failed run: https://github.com/open-telemetry/opentelemetry.io/actions/runs/18631539224/job/53116938055"