Return empty string if not docstring exists

magicli/magicli.py

@@ -170,7 +170,7 @@ def make_lines(config):
 
 def truncate_docstring(docstring):
     if not docstring:
-        return docstring
+        return ''
     truncated_docstring = []
     for line in docstring.split('\n'):
         if not line.lstrip():

tests/test_truncate_docstring.py

@@ -2,7 +2,7 @@ from magicli.magicli import truncate_docstring
 
 
 def test_no_docstring():
-    assert truncate_docstring(None) == None
+    assert truncate_docstring(None) == ''
 
 def test_padded_multiline_string():
     assert truncate_docstring("""This