tmf: Fix incorrect zoom thread time bounds when time graph view is empty

When the time graph view is empty because none of its build threads have
started yet, the time bounds still have the initial values set to [+INF,
-INF].

If a zoom thread is started at that moment, the requested start and end
time of the zoom thread are clamped to the time graph view's bounds and
are consequently set to [-INF, +INF].

This is changed so that the zoom thread does not clamp the requested
start and end time when the time graph view's bounds are not set.

Change-Id: I6c751641dab8eb4d1389adabf89c3c5a614e6b90
Signed-off-by: Patrick Tasse <patrick.tasse@gmail.com>
Reviewed-on: https://git.eclipse.org/r/77279
Reviewed-by: Hudson CI
Reviewed-by: Matthew Khouzam <matthew.khouzam@ericsson.com>
Tested-by: Jean-Christian Kouame <jean-christian.kouame@ericsson.com>