Fix config generation for non-empty source dir list

templates/config.yaml.j2

 location:
   # List of source directories to backup. Globs are expanded.
-  source_directories:  {{ borgbackup_source_dirs | to_nice_yaml }}
+{% if borgbackup_source_dirs | length == 0 %}
+  source_directories: []
+{% else %}
+  source_directories:
+{% for item in borgbackup_source_dirs %}
+    - {{ item }}
+{% endfor %}
+{% endif %}
 
   # Paths to local or remote repositories.
   repositories: