tpl/transform: Add transform.Unmarshal func

Fixes #5428

deps/deps.go

@@ -123,6 +123,9 @@ type Listeners struct {
 
 // Add adds a function to a Listeners instance.
 func (b *Listeners) Add(f func()) {
+	if b == nil {
+		return
+	}
 	b.Lock()
 	defer b.Unlock()
 	b.listeners = append(b.listeners, f)
@@ -192,6 +195,14 @@ func New(cfg DepsCfg) (*Deps, error) {
 		fs = hugofs.NewDefault(cfg.Language)
 	}
 
+	if cfg.MediaTypes == nil {
+		cfg.MediaTypes = media.DefaultTypes
+	}
+
+	if cfg.OutputFormats == nil {
+		cfg.OutputFormats = output.DefaultFormats
+	}
+
 	ps, err := helpers.NewPathSpec(fs, cfg.Language)
 
 	if err != nil {