aiohttpparser: Use double quoted empty string

This should make the PR pass the style check

webargs/aiohttpparser.py

@@ -100,7 +100,7 @@ async def parse_json(self, req, name, field):
             try:
                 json_data = await req.json()
             except json.JSONDecodeError as e:
-                if e.doc == '':
+                if e.doc == "":
                     return core.missing
                 else:
                     raise e