Fix nilpointer on ToC heading

Fixes #11843

Co-authored-by: Joe Mooring <joe.mooring@veriphor.com>

common/hreflect/helpers.go

@@ -101,6 +101,10 @@ func IsTruthfulValue(val reflect.Value) (truth bool) {
 		return
 	}
 
+	if val.Kind() == reflect.Pointer && val.IsNil() {
+		return
+	}
+
 	if val.Type().Implements(zeroType) {
 		return !val.Interface().(types.Zeroer).IsZero()
 	}

common/hreflect/helpers_test.go

@@ -22,13 +22,29 @@ import (
 	qt "github.com/frankban/quicktest"
 )
 
+type zeroStruct struct {
+	zero bool
+}
+
+func (z zeroStruct) IsZero() bool {
+	return z.zero
+}
+
 func TestIsTruthful(t *testing.T) {
 	c := qt.New(t)
 
+	var nilpointerZero *zeroStruct
+
 	c.Assert(IsTruthful(true), qt.Equals, true)
 	c.Assert(IsTruthful(false), qt.Equals, false)
 	c.Assert(IsTruthful(time.Now()), qt.Equals, true)
 	c.Assert(IsTruthful(time.Time{}), qt.Equals, false)
+	c.Assert(IsTruthful(&zeroStruct{zero: false}), qt.Equals, true)
+	c.Assert(IsTruthful(&zeroStruct{zero: true}), qt.Equals, false)
+	c.Assert(IsTruthful(zeroStruct{zero: false}), qt.Equals, true)
+	c.Assert(IsTruthful(zeroStruct{zero: true}), qt.Equals, false)
+	c.Assert(IsTruthful(nil), qt.Equals, false)
+	c.Assert(IsTruthful(nilpointerZero), qt.Equals, false)
 }
 
 func TestGetMethodByName(t *testing.T) {

markup/tableofcontents/tableofcontents_integration_test.go

@@ -121,3 +121,16 @@ CONTENT
 	b, _ = hugolib.TestE(t, files)
 	b.AssertLogMatches(`error calling ToHTML: startLevel: unable to cast "x" of type string`)
 }
+
+func TestHeadingsNilpointerIssue11843(t *testing.T) {
+	t.Parallel()
+	files := `
+-- hugo.toml --
+-- layouts/home.html --
+{{ $h := index .Fragments.HeadingsMap "bad_id" }}
+{{ if not $h }}OK{{ end }}
+`
+	b := hugolib.Test(t, files)
+
+	b.AssertFileContent("public/index.html", "OK")
+}