Revert "adds empty time value validation"

This reverts commit 7a77805.

Reverting since not required anymore to prevent ui breaking,
There's a workaround introduced with:
6d10ff7.

traceapp/vis.go

@@ -92,11 +92,6 @@ func (a *App) d3timelineInner(t *appdash.Trace, depth int) ([]timelineItem, erro
 	}
 	for _, e := range events {
 		if e, ok := e.(appdash.TimespanEvent); ok {
-			// Continue to next iteration
-			// if e.Start() or e.End() are empty time values.
-			if e.Start() == (time.Time{}) || e.End() == (time.Time{}) {
-				continue
-			}
 			start := e.Start().UnixNano() / int64(time.Millisecond)
 			end := e.End().UnixNano() / int64(time.Millisecond)
 			ts := timelineItemTimespan{