[gnumeric] so-filled: don't save an empty set of attributes.

[Morten Welinder <mortenw src gnome org>]

commit ec55d85e164f98d75a12ecce793fd48692b617dc
Author: Morten Welinder <terra gnome org>
Date:   Sun Feb 6 12:09:14 2022 -0500

    so-filled: don't save an empty set of attributes.
    
    They won't come back right, so don't.

 src/gnm-so-filled.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)
---
diff --git a/src/gnm-so-filled.c b/src/gnm-so-filled.c
index 0a940487c..80746c158 100644
--- a/src/gnm-so-filled.c
+++ b/src/gnm-so-filled.c
@@ -338,8 +338,11 @@ gnm_so_filled_write_xml_sax (SheetObject const *so, GsfXMLOut *output,
                gsf_xml_out_add_cstr (output, "Label", sof->text);
                if (sof->markup != NULL) {
                        GOFormat *fmt = go_format_new_markup (sof->markup, TRUE);
-                       gsf_xml_out_add_cstr (output, "LabelFormat",
-                                             go_format_as_XL (fmt));
+                       // Trouble: an empty markup comes back as "@" which
+                       // will not parse as markup.  Hack it for now.
+                       if (go_format_is_markup (fmt))
+                               gsf_xml_out_add_cstr (output, "LabelFormat",
+                                                     go_format_as_XL (fmt));
                        go_format_unref (fmt);
                }
        }