promtool: Fix panic on extended tsdb analyze (#13976)

Currently, running promtool tsdb analyze with the --extended flag
will cause an 'index out of range' error if running it
against a block that does not have any native histogram chunks.

This change ensures that promtool won't try to display data that doesn't exist.

Signed-off-by: Will Hegedus <whegedus@linode.com>

cmd/promtool/tsdb.go

@@ -838,6 +838,10 @@ func backfillOpenMetrics(path, outputDir string, humanReadable, quiet bool, maxB
 }
 
 func displayHistogram(dataType string, datas []int, total int) {
+	if len(datas) == 0 {
+		fmt.Printf("%s: N/A\n\n", dataType)
+		return
+	}
 	slices.Sort(datas)
 	start, end, step := generateBucket(datas[0], datas[len(datas)-1])
 	sum := 0