EB: Push metadata from Manage should ensure IdP has discovery information

[johanib]

Based on: OpenConext/OpenConext-devconf#60

Manage migrated the IdP name to the discovery name fields.
However, if for some reason manage does not post the discoveries in the metadata push to EB, EB will still accept the metadata push.

However, when rendering the wayf, EB will crash with a type error (NULL passed, string expected).

Change request:
EB should not accept the Metadata push if the discovery name is empty (or fallback to the IdP name during metadata push).