7z gives an error when using the header encryptions switch (-mhe=on|off)
with archive types that don't support header encryption such as zip.
This results in Ark failing to create new zip archives with cli7z.

This is now fixed by setting $EncryptHeaderSwitch to empty when header
encryption is off, instead of -mhe=off.

This patch also fixes CreateDialog::isHeaderEncryptionChecked() to only
return true if encryptHeaderCheckBox is both enabled and checked,
instead of just checked.

REVIEW: 125507