fix(analyze): write an undefined paired p-value as null, not as NaN

A paired contrast where censoring orders no pair has no p-value, and JSON has
no NaN, so --json failed with 'marshal summary: json: unsupported value: NaN'
and wrote no summary at all after printing a complete report. The two fields
join the medians and the rates already carried as pointers, undefined reading
as null in the summary and n/a in the report.

Reachable since a clean run started being censored where it stopped: an arm
the wall clock stops before its partner ever violates orders nothing.

cmd/internal-tools/analyze/report.go

@@ -149,7 +149,7 @@ func writePaired(out io.Writer, comparison pairedComparison) {
 		formatStepDifference(comparison.MedianDifference), comparison.BothViolated, comparison.Sign, comparison.A12)
 	fmt.Fprintf(out, "sign test over the %d ordered pair(s), p %s, holm p %s\n",
 		comparison.FirstSooner+comparison.SecondSooner,
-		formatPValue(comparison.PValue), formatPValue(comparison.HolmPValue))
+		formatOptionalPValue(comparison.PValue), formatOptionalPValue(comparison.HolmPValue))
 	if len(comparison.UnpairedSeeds) > 0 {
 		fmt.Fprintf(out, "%d seed(s) usable in one arm only and left out of the pairing: %v\n",
 			len(comparison.UnpairedSeeds), comparison.UnpairedSeeds)
@@ -209,6 +209,13 @@ func formatSingletons(summary armSummary) string {
 	return fmt.Sprintf("%d/%d (%.3f)", summary.SingletonDefects, summary.DistinctDefects, *summary.SingletonFraction)
 }
 
+func formatOptionalPValue(value *float64) string {
+	if value == nil {
+		return "n/a"
+	}
+	return formatPValue(*value)
+}
+
 func formatPValue(value float64) string {
 	switch {
 	case math.IsNaN(value):