bootflow: Show an empty filename when there is none

At present 'bootflow list' shows <NULL> for the filename when it is not
present. Show an empty string instead, since that is more user-friendly.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/cmd/bootflow.c b/cmd/bootflow.c
index 3c3abaf..300ad3a 100644
--- a/cmd/bootflow.c
+++ b/cmd/bootflow.c
@@ -71,7 +71,7 @@
 	printf("%3x  %-11s  %-6s  %-9.9s %4x  %-25.25s %s\n", index,
 	       bflow->method->name, bootflow_state_get_name(bflow->state),
 	       bflow->dev ? dev_get_uclass_name(dev_get_parent(bflow->dev)) :
-	       "(none)", bflow->part, bflow->name, bflow->fname);
+	       "(none)", bflow->part, bflow->name, bflow->fname ?: "");
 	if (errors)
 		report_bootflow_err(bflow, bflow->err);
 }