Improve nullability for `getSession(*)` in `MockHttpServletRequest`

[OrangeDog]

MockHttpServletRequest.getSession() will always create a new session if there isn't one, and so can never return null.

Similarly, getSession(boolean) should have a @Contract("true -> !null").

[sbrannen]

Good catch! 👍

We'll sort this out for 7.0.9.