_win_print: Guard against time being null

This is encountered when biboumi is used.
Example: `/join #debian-next%irc.oftc.net@biboumi.lebihan.pl`
It seems then time can be null.

g_date_time_format() will not work in this case although time_pref will
not be "off". So let's not call g_date_time_format() in this case. But
treat it like time is set to off.

However message reflection will not work properly with biboumi.
Probably we dont get origin-id.

Fix #1230

src/ui/window.c

@@ -1423,7 +1423,7 @@ _win_print(ProfWin *window, const char show_char, int pad_indent, GDateTime *tim
     }
 
     gchar *date_fmt = NULL;
-    if (g_strcmp0(time_pref, "off") == 0) {
+    if (g_strcmp0(time_pref, "off") == 0 || time == NULL) {
         date_fmt = g_strdup("");
     } else {
         date_fmt = g_date_time_format(time, time_pref);