cmd/trace: avoid emitting traceview slice with 0 duration
The trace viewer interprets the slice as a non-terminating
time interval which is quite opposit to what trace records indicate
(i.e., almostly immediately terminating time interval).
As observed in the issue #24663 this can result in quite misleading
visualization of the trace.
Work around the trace viewer's issue by setting a small value
(0.0001usec) as the duration if the time interval is not positive.