Update containers.build.yaml

Signed-off-by: Kjeld Schouten <info@kjeldschouten.nl>

.github/workflows/containers.build.yaml

@@ -45,6 +45,10 @@ jobs:
       - id: reduce
         run: |
           CONTAINERS=$(jq --raw-output '.changed_files | fromjson | .[] |= sub("(?<filepath>/?containers/(?<first_directory>(?<root1>[/]?)[^/]+/)(?<second_directory>(?<root2>[/]?)[^/]+)(?<extra_paths>/[^/]+)*)"; "\(.second_directory)") | unique' changes.json)
+          if [ -z "$CONTAINERS" ]; then
+            echo "No containers found."
+            CONTAINERS="[]"
+          fi
           echo ::set-output name=containers::${CONTAINERS}
 
   hadolint: