Fix initial trace time range.

Fix initial trace time range.

The initial start time of TmfTrace is set to BIG_BANG so that the
initial time range is equal to NULL_RANGE. This allows to prevent
unnecessary histogram requests when a trace is opened.

Since its base class defaults to NULL_RANGE, it is no longer necessary
to explicitly set it in the TmfExperiment constructor.

Unit tests updated accordingly.

A fix is implemented in custom traces for possible NullPointerException
that uncovered this issue.

Change-Id: Ib1e49f372e1c5b1eaa3aecb972cc2b3830b3fbe3
Reviewed-on: https://git.eclipse.org/r/8603
Tested-by: Hudson CI
Reviewed-by: Alexandre Montplaisir <alexmonthy@voxpopuli.im>
IP-Clean: Alexandre Montplaisir <alexmonthy@voxpopuli.im>
Tested-by: Alexandre Montplaisir <alexmonthy@voxpopuli.im>
Reviewed-by: Bernd Hufmann <bhufmann@gmail.com>
IP-Clean: Bernd Hufmann <bhufmann@gmail.com>
Reviewed-by: Patrick Tasse <patrick.tasse@gmail.com>
IP-Clean: Patrick Tasse <patrick.tasse@gmail.com>
Tested-by: Patrick Tasse <patrick.tasse@gmail.com>