util/limiter: don't panic when dumping a new Limiter

Fixes #18439

Signed-off-by: Josh Bleecher Snyder <josharian@gmail.com>

util/limiter/limiter.go

@@ -187,6 +187,9 @@ func (lm *Limiter[K]) collectDump(now time.Time) []dumpEntry[K] {
 	lm.mu.Lock()
 	defer lm.mu.Unlock()
 
+	if lm.cache == nil {
+		return nil
+	}
 	ret := make([]dumpEntry[K], 0, lm.cache.Len())
 	lm.cache.ForEach(func(k K, v *bucket) {
 		lm.updateBucketLocked(v, now) // so stats are accurate

util/limiter/limiter_test.go

@@ -5,6 +5,7 @@ package limiter
 
 import (
 	"bytes"
+	"io"
 	"strings"
 	"testing"
 	"time"
@@ -175,6 +176,10 @@ func TestDumpHTML(t *testing.T) {
 	}
 }
 
+func TestDumpHTMLEmpty(t *testing.T) {
+	new(Limiter[string]).DumpHTML(io.Discard, false) // should not panic
+}
+
 func allowed(t *testing.T, limiter *Limiter[string], key string, count int, now time.Time) {
 	t.Helper()
 	for i := range count {