ci(code-coverage-deploy): only load historical lcov if it exists

The code-coverage-deploy workflow was failing, as it was trying to use
historical lcov when it did not exist. This patch makes this step
conditional on whether historical lcov exists or not.

https://github.com/conjure-cp/conjure-oxide/actions/runs/11146109623/job/30977305993

.github/workflows/code-coverage-deploy.yml

@@ -234,6 +234,7 @@ jobs:
  })
  
  - name: Download the previous lcov.info (historical) file
+ if: steps.find_artifacts.outputs.result != '[]'
  uses: actions/download-artifact@v4
  with: |
  name: code-coverage-${{ env.num }}-${{ env.previous_artifact_id }}