Avoid debug message when there is no session manager

See #16024

src/gtk/utilsgtk.cpp

@@ -253,8 +253,12 @@ static wxString GetSM()
                                          WXSIZEOF(smerr), smerr);
 
     if ( !smc_conn )
+    {
+        // Don't report error if there is no session manager at all
+        if (getenv("SESSION_MANAGER"))
         {
             wxLogDebug("Failed to connect to session manager: %s", smerr);
+        }
         return wxEmptyString;
     }