fix: Deal with empty {{ content }}

Author: SilverRainZ

src/sphinxnotes/project/templates/autoconfval.rst

@@ -18,6 +18,6 @@
    {{ line }}
    {% endfor %}
 
-{% for line in content.split('\n') -%}
+{% for line in (content or '').split('\n') -%}
 {{ line }}
 {% endfor %}

src/sphinxnotes/project/templates/autoobj.rst

@@ -34,6 +34,6 @@ Cross-reference roles for referencing a ``{{ dispname }}`` object:
 {% endif %}
 {% endfor %}
 
-{% for line in content.split('\n') -%}
+{% for line in (content or '').split('\n') -%}
 {{ line }}
 {% endfor %}