Response with unvalued AudienceRestriction (Condition) Handling

[peppelinux]

This PR prevents this exception

saml2/response.py", line 219, in for_me
    if audience.text.strip() == myself:
AttributeError: 'NoneType' object has no attribute 'strip'

When a SP get a Response with an unvalued AudienceRestriction (Condition) like this:

        <saml:Conditions NotBefore="2021-01-24T13:57:51Z" NotOnOrAfter="2021-01-24T14:02:44Z">
            <saml:AudienceRestriction>
                <saml:Audience/>
            </saml:AudienceRestriction>
        </saml:Conditions>

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