[gnumeric] Sheet Object: evidently format for image writing can be null.

[Morten Welinder <mortenw src gnome org>]

commit 04e86e69180913ec952016af0c3758cebc3ac93f
Author: Morten Welinder <terra gnome org>
Date:   Wed Nov 6 18:56:56 2019 -0500

    Sheet Object: evidently format for image writing can be null.
    
    t6518-objects.pl was not happy.

 src/sheet-object.c | 6 ++----
 1 file changed, 2 insertions(+), 4 deletions(-)
---
diff --git a/src/sheet-object.c b/src/sheet-object.c
index 2f1aed8ce..e2c661601 100644
--- a/src/sheet-object.c
+++ b/src/sheet-object.c
@@ -1718,7 +1718,7 @@ sheet_object_get_target_list (SheetObject const *so)
 /**
  * sheet_object_write_image:
  * @so: #SheetObject
- * @format: image format to use
+ * @format: (nullable): image format to use
  * @resolution: export resolution
  * @output: destination
  * @err: (out) (optional) (nullable): error indication
@@ -1731,7 +1731,6 @@ sheet_object_write_image (SheetObject const *so, char const *format, double reso
                          GsfOutput *output, GError **err)
 {
        g_return_if_fail (GNM_IS_SO_IMAGEABLE (so));
-       g_return_if_fail (format != NULL);
        g_return_if_fail (GSF_IS_OUTPUT (output));
 
        GNM_SO_IMAGEABLE_CLASS (so)->write_image (so, format, resolution,
@@ -1742,7 +1741,7 @@ sheet_object_write_image (SheetObject const *so, char const *format, double reso
 /**
  * sheet_object_save_as_image:
  * @so: #SheetObject
- * @format: image format to use
+ * @format: (nullable): image format to use
  * @resolution: export resolution
  * @url: destination url
  * @err: (out) (optional) (nullable): error indication
@@ -1760,7 +1759,6 @@ sheet_object_save_as_image (SheetObject const *so,
        GsfOutput *dst;
 
        g_return_if_fail (GNM_IS_SO_IMAGEABLE (so));
-       g_return_if_fail (format != NULL);
        g_return_if_fail (url != NULL);
 
        dst = go_file_create (url, err);