Handle commands without a docstring

'docutils.statemachine.string2lines' doesn't handle 'None', which is
what 'command.help' returns if a command does not have a docstring (from
which the help string is derived).

Signed-off-by: Stephen Finucane <stephen@that.guru>
Closes-Bug: #3

sphinx_click/ext.py

@@ -236,11 +236,12 @@ def _generate_nodes(self, name, command, parent=None, show_nested=False):
         # We parse this as reStructuredText, allowing users to embed rich
         # information in their help messages if they so choose.
 
-        for line in statemachine.string2lines(
-                ctx.command.help, tab_width=4, convert_whitespace=True):
-            result.append(line, source_name)
+        if ctx.command.help:
+            for line in statemachine.string2lines(
+                    ctx.command.help, tab_width=4, convert_whitespace=True):
+                result.append(line, source_name)
 
-        result.append('', source_name)
+            result.append('', source_name)
 
         # Summary