Make sure that ij-legacy's EventDelegator knows the legacy service

The recent changes in scijava-common allow Contextuals to have
@Parameters that are automatically initialized upon setContext(), so we
changed the EventDelegator accordingly.

The only problem with that was that we called setContext() indirectly
from DefaultLegacyService's initialize() method, therefore the context
did not have a legacy service yet.

Let's just play things safely and ask the current context again when we
do not have a legacy service.

This happens to fix http://fiji.sc/bugzilla/show_bug.cgi?id=662,
reported by Jan Eglinger.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>