Fall back to boolean instead of null

[dnwjn]

Hi!

This change fixes the following error we receive when navigating to the CP /navigation/<nav_name>:

Statamic\Support\Str::bool(): Argument #1 ($value) must be of type bool, null given, called in /<redacted>/storage/framework/views/f7f7a695478ceb1af2141e5f7389a1b0.php on line 21 (View: /<redacted>/vendor/statamic/cms/resources/views/navigation/show.blade.php)

This started happening after updating Composer dependencies, limited to patch and minor versions. The change in this PR is a possible fix, but that depends on if the default is considered false. Otherwise another option could be:

// vendor/statamic/cms/src/Structures/Nav.php

public function canSelectAcrossSites($canSelect = null)
{
    return $this
        ->fluentlyGetOrSet('canSelectAcrossSites')
+        ->setter(function ($canSelect) {
+            return (bool) $canSelect;
+        })
        ->args(func_get_args());
}

Let me know what you think!

[ryanmitchell]

Im happy with this, thanks!