Fix extract_function when the extracted code defines no variable

If the extracted lines define no name, the new function ended with a
bare "return " and the call site became " = func(v1)". Both are
invalid Python.

Only add the final return line and the assignment when there is at
least one variable to return. Otherwise call the function as a
statement.

AUTHORS.txt

@@ -71,6 +71,7 @@ Code Contributors
 - Dhanraj E (@Dhanu-dynamic) <dhanraj@labs.digiscrypt.com>
 - Charan Rathore (@charan-rathore)
 - Abdullah Ahmed (@abdull-ah-med)
+- spicysharkco (@spicysharkco)
 
 And a few more "anonymous" contributors.