Signature "0" cannot be previewed

includes/ucp/ucp_profile.php only passes the signature itself ('SIGNATURE_PREVIEW' => $signature_preview) to the template, but not whether the preview button was hit or not (it doesn't pass $preview to the template).

Thus the templates always have to use something like

<!-- IF SIGNATURE_PREVIEW != '' -->

instead of

<!-- IF SIGNATURE_PREVIEW -->

to not disallow "0".

This affects both, prosilver and subsilver2.