Check if string_names is None before returning string_names (#1708)

* Check if string is None before using string_names

* Add test asserting None string_names returns an empty list

* Remove whitespace to pass flake8

* Add name to authors.txt

Co-authored-by: Leo Ryu <leo@episci.com>

AUTHORS.txt

@@ -60,6 +60,7 @@ Code Contributors
 - Max Mäusezahl (@mmaeusezahl) <maxmaeusezahl@googlemail.com>
 - Vladislav Serebrennikov (@endilll)
 - Andrii Kolomoiets (@muffinmad)
+- Leo Ryu (@Leo-Ryu)
 
 And a few more "anonymous" contributors.