aiohttp response body check none fix

Author: p1c2u

Diff for: openapi_core/contrib/aiohttp/responses.py

@@ -14,6 +14,8 @@ def __init__(self, response: web.Response):
 
     @property
     def data(self) -> str:
+        if self.response.body is None:
+            return ""
         if isinstance(self.response.body, bytes):
             return self.response.body.decode("utf-8")
         assert isinstance(self.response.body, str)