Bugfix: Fix a bug when some fields where not retrieved from OIDC Discovery

Problem when requesting the OIDC discovery was fixed. Some fields were handled
as mandatory, but they are recommended according to the docs. It prevented from
a proper login. Now it is possible to login as expected when some fields are not retrieved.

https://github.com/owncloud/android/pull/3202
https://github.com/owncloud/android-library/pull/392
