Use empty() to avoid undefined var error during bootstrap

[volkjohn]

On a FreePBX 17 PHP 8.2 system, I was getting an undefined variable (username) error on line 21 of gui_auth.php when called by bootstrap.php.

Using empty() is a safer way to check for the existence of the username and password variable.

I'm running this in production with no issue.

[sangoma-oss-cla[bot]]

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