Fix crash when running with optimization (#2160)

Co-authored-by: Michał Krassowski <[email protected]>

Author: QuLogic

nbconvert/exporters/templateexporter.py

@@ -151,7 +151,11 @@ class TemplateExporter(Exporter):
     """
 
     # finish the docstring
-    __doc__ = __doc__.format(filters="- " + "\n    - ".join(sorted(default_filters.keys())))
+    __doc__ = (
+        __doc__.format(filters="- " + "\n    - ".join(sorted(default_filters.keys())))
+        if __doc__
+        else None
+    )
 
     _template_cached = None