tpl: Get rid of the custom template truth logic

Fixes #6615

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
+}