Avoid nilpointer on no File on Page

Fixes #5781

source/fileInfo.go

@@ -53,6 +53,8 @@ type fileOverlap interface {
 	// Lang is the language code for this page. It will be the
 	// same as the site's language code.
 	Lang() string
+
+	IsZero() bool
 }
 
 type FileWithoutOverlap interface {
@@ -187,6 +189,10 @@ func (fi *FileInfo) Open() (hugio.ReadSeekCloser, error) {
 	return f, err
 }
 
+func (fi *FileInfo) IsZero() bool {
+	return fi == nil
+}
+
 // We create a lot of these FileInfo objects, but there are parts of it used only
 // in some cases that is slightly expensive to construct.
 func (fi *FileInfo) init() {