Set user_tty to "unknown" if there is no tty, like sudo 1.7 does

(it is used when logging).  Note that user_ttypath will still be
NULL if there is no tty.

plugins/sudoers/sudoers.c

@@ -1314,6 +1314,8 @@ deserialize_info(char * const settings[], char * const user_info[])
 	    continue;
 	}
     }
+    if (user_tty == NULL)
+	user_tty = "unknown"; /* user_ttypath remains NULL */
 
 #undef MATCHES
     return flags;