code quality: fix $force param default value to align with doctype in force_ssl_content()

[justlevine]

This PR changes the default value for the force_sssl_content() 's $force parameter, to correctly align with the existing doc type.

Since the function internally uses a loose ! $force falsey check, there is no change to any behavior.

While this issue was surfaced via PHPStan in #7619 (trac: https://core.trac.wordpress.org/ticket/61175 ), it can be remediated independently of that ticket.

Trac ticket: https://core.trac.wordpress.org/ticket/52217

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