hugolib: Avoid nilpointer in absoluteSourceRef

hugolib/page.go

@@ -1883,10 +1883,12 @@ func (p *Page) FullFilePath() string {
 // For pages that do not (sections witout content page etc.), it returns the
 // virtual path, consistent with where you would add a source file.
 func (p *Page) absoluteSourceRef() string {
+	if p.Source.File != nil {
 		sourcePath := p.Source.Path()
 		if sourcePath != "" {
 			return "/" + filepath.ToSlash(sourcePath)
 		}
+	}
 
 	if len(p.sections) > 0 {
 		// no backing file, return the virtual source path