Ensure to always read value from settings

Summary:
Unchecked checkboxes in forms don't produce values.

This change fixes the following issue:

Undefined array key "SmartlineMethod"
in /var/51-wwwroot/zed/includes/settings/page.php on line 128

Ref T112

Test Plan: Tested on zed51. updating SmartLine settings

Reviewers: dereckson

Reviewed By: dereckson

Maniphest Tasks: T112

Differential Revision: https://code.zed.dereckson.be/D25