When that mode is disabled, then actually stop morlocking.

Suggested-by: Daniel Mendler <mail@daniel-mendler.de>

Before this, font-locking was not properly refreshed when toggling
the global mode.

Right now the completing-read advice does not cover other places where
the empty symbol appears, e.g., `completion-at-point'.  Therefore
address the problem on a slightly lower level, by using an advice on
`all-completions', directly where the completion candidates are
generated.  This advice is simpler since it removes the empty string
from the candidates instead of mutating the obarray.