IssueInstant attribute prior to IssueInstant of the Request
A PySAML2 SP seems to work properly even if it receive a Response like the following:
````
<samlp:Response Destination="http://sp1.testunical.it:8000/saml2/acs/" ID="_c19ec300-6e47-4ea7-86f0-a82bc6747440" InResponseTo="id-eamgZfiD3S1TDNg1a" IssueInstant="2021-01-24T01:05:01Z" Version="2.0" xmlns:saml="urn:oasis:names:tc:SAML:2.0:assertion" xmlns:samlp="urn:oasis:names:tc:SAML:2.0:protocol">

[...]

<saml:Assertion ID="_b431a7c8-4ad3-479a-b867-a5d9625e98bb" IssueInstant="2018-09-06 16:00" Version="2.0" xmlns:xs="http://www.w3.org/2001/XMLSchema" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
````

Same behaviour with a IssueIstant prior to the AuthnRequest 

````
<samlp:Response Destination="http://sp1.testunical.it:8000/saml2/acs/" ID="_cd3bb191-9934-485b-98ec-87676cc1e023" InResponseTo="id-VOHjtNzUB1vnQ2i90" IssueInstant="2018-01-01T00:00:00Z" Version="2.0" xmlns:saml="urn:oasis:names:tc:SAML:2.0:assertion" xmlns:samlp="urn:oasis:names:tc:SAML:2.0:protocol">
````
diff --git a/src/saml2/response.py b/src/saml2/response.py
index 26963a04..9d4021ee 100644
--- a/src/saml2/response.py
+++ b/src/saml2/response.py
@@ -1044,7 +1044,7 @@ class AuthnResponse(StatusResponse):
             logger.error("Verification error on the response: %s", err)
             raise
         else:
-            if res is None:
+            if not res:
                 return None
 
         if not isinstance(self.response, samlp.Response):
['src/saml2/response.py']