Fix "Undefined index: HTTPS" warning notice when not using HTTPS

[geominorai]

When the page is loaded through HTTP instead of HTTPS the login page shows a PHP notice on the page:
Undefined index: HTTPS

Description

Added isset() check for $_SERVER['HTTPS'] and handle case where a web server config has the value set to off.

Motivation and Context

This error is hidden from public view with display_errors = Off in php.ini but should still be corrected.

How Has This Been Tested?

Tested on Linux with Apache 2.4.29 / PHP 7.2.24
Page now loads without the error. Steam OAuth login redirects appropriately for both HTTP and HTTPS.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.