tmf: Handle empty traces with the abstract state provider

tmf: Handle empty traces with the abstract state provider

Traces with state systems using the AbstractTmfStateProvider (ie,
all of them) would not get closed correctly if no event was ever
received in the provider - currentEvent would remain null, and the
closing step would be skipped.

This is probably an artifact back from the time where history backends
were optional. Now the closing step should always be run, and if we do
not have a valid end time to define, simply use 0. The dummy state
system will go from MIN_VALUE to 0, and since it will have absolutely
no data in it, it really doesn't matter.

Change-Id: Ia36d7bb2cce0b5712825043c7d9becd85cd7ec35
Signed-off-by: Alexandre Montplaisir <alexmonthy@voxpopuli.im>
Reviewed-on: https://git.eclipse.org/r/21576
Tested-by: Hudson CI
Reviewed-by: Matthew Khouzam <matthew.khouzam@ericsson.com>
IP-Clean: Matthew Khouzam <matthew.khouzam@ericsson.com>