Only archive error content that is not NULL.

html.c

@@ -1693,8 +1693,9 @@ xs_html *html_entry(snac *user, xs_dict *msg, int read_only,
         const char *content = xs_dict_get(msg, "content");
 
         if (xs_type(content) != XSTYPE_STRING) {
-            srv_archive_error("unexpected_content_xstype",
-                "content field type", xs_stock(XSTYPE_DICT), msg);
+            if (!xs_is_null(content))
+                srv_archive_error("unexpected_content_xstype",
+                    "content field type", xs_stock(XSTYPE_DICT), msg);
 
             content = "";
         }