OutputPage: Set empty modules to state 'ready' instead of 'missing'.

OutputPage: Set empty modules to state 'ready' instead of 'missing'.

Some cases were already covered by I595b3af190, but in the case
where the module is disabled entirely it was setting it to missing.

This is inconsistent with the behaviour of load.php (which will implement
an empty module resulting in state=ready) and confusing as it
implies there is a problem (a missing module) whereas in fact it
isn't a problem. Modules that depend on it to allow customization
wouldn't load as it will fail on missing dependencies.

Also makes asserting problems (bug 44299) hard as it would have
to whitelist state 'missing' for user/site/user.groups.

Bug: 46857
Change-Id: I46b5bc6f714abe4b66186d7b7a14c69195f00ee1