Handle empty default landing page (#209)

Otherwise there will be infinite redirects.

.github/workflows/pkgdown.yaml

@@ -289,11 +289,23 @@ jobs:
           path: ${{ github.event.repository.name }}
           ref: "gh-pages"
 
+      - name: Normalize inputs 📄
+        id: normalize
+        run: |
+          DEFAULT_LANDING_PAGE="${{ inputs.default-landing-page }}"
+          if [ "$DEFAULT_LANDING_PAGE" == "" ]
+          then {
+            DEFAULT_LANDING_PAGE=main
+          }
+          fi
+          echo "default-landing-page=$DEFAULT_LANDING_PAGE" >> "$GITHUB_ENV"
+        shell: bash
+
       - name: Create and publish docs ↗️
         uses: insightsengineering/r-pkgdown-multiversion@v2
         with:
           path: ${{ github.event.repository.name }}
-          default-landing-page: ${{ inputs.default-landing-page }}
+          default-landing-page: ${{ env.default-landing-page }}
           refs-order: ${{ inputs.refs-order }}
           latest-tag-alt-name: ${{ inputs.latest-tag-alt-name }}
           release-candidate-alt-name: ${{ inputs.release-candidate-alt-name }}