ProtectSiteForm: don't pass null to htmlspecialchars()

To fix PHP 8.1 emitting a warning.
This fixes the config value default for $wgProtectSiteDefaultTimeout. Currently is used wrong capitalisation, so the global was returning null when passed to the textbox() function $value in ProtectSiteForm.

Change-Id: I2374ddbb9d16315c59ab95c3b9a66c1d1f07f359
(cherry picked from commit 906070a55bb339d4914b52f7181d0bc168680c65)