Changes

tmf: Bug 579358: Clearing filter from editor doesn't update time chart

Restore broadcasting of TmfEventFilterAppliedSignal with null filter
when all filters are cleared. Tag the filter as @Nullable in the signal.

Change-Id: Ib964eaac7cad5fd97fd822d92d750c4e3eaad15c
Signed-off-by: Patrick Tasse <patrick.tasse@gmail.com>
Reviewed-on: https://git.eclipse.org/r/c/tracecompass/org.eclipse.tracecompass/+/192227
Tested-by: Trace Compass Bot <tracecompass-bot@eclipse.org>
Tested-by: Bernd Hufmann <bernd.hufmann@ericsson.com>
Reviewed-by: Marco Miller <marco.miller@ericsson.com>
Reviewed-by: Bernd Hufmann <bernd.hufmann@ericsson.com>