app: set the blank font name for the standard font instead of 'Standard'

to fix the issue when 'Standard' can be actual font from the list
or a font which starts with. (ie. Standard Symbols PS)

Fixes: #3104 (GNOME/Gimp tracker)

app/text/gimpfont.c

@@ -347,7 +347,7 @@ gimp_font_get_standard (void)
     {
       g_set_weak_pointer (&standard_font,
                           g_object_new (GIMP_TYPE_FONT,
-                                        "name", "Standard",
+                                        "name", "",
                                         NULL));;
 
       gimp_data_clean (standard_font);