opt_constant_string_type can crash

[jrmbchtl]

cst_string_node.text can be None, so trying to do
member = cst_string_node.text.lower() will crash in this case

Fix:
just insert
if cst_string_node.text is None: continue
inbetween
for cst_string_node in node.findall("StringConstantExpressionAst"):
and
member = cst_string_node.text.lower()

[thewhiteninja]

fixed in cc322c2
Thanks for the bug report!