Uodate workflow to not fail if there is no theme preview

Author: mathoudebine

.github/workflows/themes-screenshot-on-pr.yml

@@ -55,9 +55,11 @@ jobs:
             cp screencap.png "screenshot-$theme.png"
 
             # Compare with original theme preview to detect changes, generate a diff
-            python3 tools/compare-images.py "screenshot-$theme.png" "$dir/preview.png" "diff-$theme.png"
-            if [ -f "diff-$theme.png" ]; then
-              echo "::warning::$theme theme rendering has changed, check if it is intentional or a side-effect. A diff is included in the job artifacts."
+            if [ -f "$dir/preview.png" ]; then
+              python3 tools/compare-images.py "screenshot-$theme.png" "$dir/preview.png" "diff-$theme.png"
+              if [ -f "diff-$theme.png" ]; then
+                echo "::warning::$theme theme rendering has changed, check if it is intentional or a side-effect. A diff is included in the job artifacts."
+              fi
             fi
           fi
         done