cmd/trace/v2: make the -pprof actually useful

In both the v1 and v2 cmd/trace, pprofMatchingGoroutines will generate
no output at all if the filter name passed to it is the empty string.

This is rather pointless because there are at least two places where we
don't pass a name to filter. Modify pprofMatchingGoroutines to include
*all* goroutines in the trace if the name to filter by is not specified.

For #66782.

Change-Id: I6b72298d676bc93892b075a7426e6e56bc6656c7
Reviewed-on: https://go-review.googlesource.com/c/go/+/578356
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Carlos Amedee <[email protected]>
Auto-Submit: Michael Knyszek <[email protected]>

src/cmd/trace/v2/pprof.go

@@ -48,7 +48,7 @@ func pprofByRegion(compute computePprofFunc, t *parsedTrace) traceviewer.Profile
 func pprofMatchingGoroutines(name string, t *parsedTrace) (map[tracev2.GoID][]interval, error) {
 	res := make(map[tracev2.GoID][]interval)
 	for _, g := range t.summary.Goroutines {
-		if g.Name != name {
+		if name != "" && g.Name != name {
 			continue
 		}
 		endTime := g.EndTime