Server error from api_get_session_materials() when session has no agenda

[jennifer-richards]

Describe the issue

The api_get_session_materials() method returns a url for the Session by calling session.agenda().get_href(). If the Session does not yet have an agenda, this causes a server error because agenda() returns None.

Need to check that the front end calling this method can deal with a null there and add a guard before calling get_href().

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