tpl: Get rid of the custom template truth logic

Fixes #6615

tpl/internal/go_templates/texttemplate/exec.go

@@ -307,7 +307,7 @@ func IsTrue(val interface{}) (truth, ok bool) {
 	return isTrue(reflect.ValueOf(val))
 }
 
-func isTrue(val reflect.Value) (truth, ok bool) {
+func isTrueOld(val reflect.Value) (truth, ok bool) {
 	if !val.IsValid() {
 		// Something like var x interface{}, never set. It's a form of nil.
 		return false, true

tpl/internal/go_templates/texttemplate/hugo_template.go

@@ -17,6 +17,8 @@ import (
 	"io"
 	"reflect"
 
+	"github.com/gohugoio/hugo/common/hreflect"
+
 	"github.com/gohugoio/hugo/tpl/internal/go_templates/texttemplate/parse"
 )
 
@@ -301,3 +303,7 @@ func (s *state) evalCall(dot, fun reflect.Value, node parse.Node, name string, a
 	}
 	return v
 }
+
+func isTrue(val reflect.Value) (truth, ok bool) {
+	return hreflect.IsTruthfulValue(val), true
+}