foce_ssl_admin/login don't work if the argument is false

[nbachiyski]

Expected: force_ssl_admin() to set the static var inside the function to false.

Actual: nothing happens.

The condition on which the value is changed is if ( '' != $force ), which can't be fulfilled by any false value. The condition is equal to if ( false != $force).

Here is a patch, which uses null as a default value and checks using is_null(). Similar effect can be accomplished by using !==, but the prefer seems to convey better the meaninglessness of the default value.

[ryan]

(In [11903]) Allow force_ssl_admin() to properly accept false as a value. Props nbachiyski. fixes #10706