tmf: Change getTracesToBuild trace parameter to be @Nullable

tmf: Change getTracesToBuild trace parameter to be @Nullable

The default implementation already handles a null trace, and
VirtualMachineView overrides it as @Nullable.

This allows subclasses to call the method with the return value of
getTrace(), which can be null, without needing a null check and get an
empty list back.

Change-Id: Ie7cb5c79bb841796be4e20fcafedb43ad8b2c839
Signed-off-by: Patrick Tasse <patrick.tasse@gmail.com>
Reviewed-on: https://git.eclipse.org/r/78104
Reviewed-by: Hudson CI
Reviewed-by: Bernd Hufmann <bernd.hufmann@ericsson.com>
Tested-by: Bernd Hufmann <bernd.hufmann@ericsson.com>