CI: fix crash when no diff

.github/workflows/sync-main.yml

@@ -49,12 +49,21 @@ jobs:
               exit 1
               ;;
           esac
+      - name: Check for changes
+        run: |
+          git fetch origin main "${{ steps.branch.outputs.source }}"
+          HAS_CHANGES=true
+          if git diff --quiet \
+            "origin/main...origin/${{ steps.branch.outputs.source }}"; then
+            HAS_CHANGES=false
+          fi
 
       - name: Check for existing PR
         id: existing
         env:
           GH_TOKEN: ${{ secrets.GITHUB_TOKEN }}
         run: |
+          if $HAS_CHANGES; then
             PR_NUMBER=$(gh pr list \
               --base main \
               --head "${{ steps.branch.outputs.source }}" \
@@ -63,6 +72,7 @@ jobs:
               --jq '.[0].number // empty')
 
             echo "number=$PR_NUMBER" >> "$GITHUB_OUTPUT"
+          fi
 
       - name: Create PR
         id: create
@@ -70,6 +80,7 @@ jobs:
         env:
           GH_TOKEN: ${{ secrets.GITHUB_TOKEN }}
         run: |
+          if $HAS_CHANGES; then
             gh pr create \
               --base main \
               --head "${{ steps.branch.outputs.source }}" \
@@ -84,22 +95,26 @@ jobs:
               --jq '.[0].number')
 
             echo "number=$PR_NUMBER" >> "$GITHUB_OUTPUT"
-
+          fi
       - name: Determine PR number
         id: pr
         run: |
+          if $HAS_CHANGES; then
             if [ -n "${{ steps.existing.outputs.number }}" ]; then
               echo "number=${{ steps.existing.outputs.number }}" >> "$GITHUB_OUTPUT"
             else
               echo "number=${{ steps.create.outputs.number }}" >> "$GITHUB_OUTPUT"
             fi
+          fi
 
       - name: Enable auto-merge
         if: steps.pr.outputs.number != ''
         env:
           GH_TOKEN: ${{ secrets.GITHUB_TOKEN }}
         run: |
+          if $HAS_CHANGES; then
             gh pr merge \
               "${{ steps.pr.outputs.number }}" \
               --auto \
               --merge
+          fi