tmf.ui: fix Bug 479996 infinite loop

tmf.ui: fix Bug 479996 infinite loop

Opening an empty trace called queryFullStates an infinite number
of times resulting in OOM. Checking the arguments will avoid
this.

Bug 479996

Change-Id: I0ac29da7a883d154fffb5c4d24aa50db32796a6f
Signed-off-by: Loïc Prieur-Drevon <loic.prieurdrevon@gmail.com>
Reviewed-on: https://git.eclipse.org/r/86862
Reviewed-by: Hudson CI
Reviewed-by: Matthew Khouzam <matthew.khouzam@ericsson.com>
Reviewed-by: Patrick Tasse <patrick.tasse@gmail.com>
Tested-by: Patrick Tasse <patrick.tasse@gmail.com>