is Signed
Returns if this form element is digitally signed. Note that this DOES NOT check if the signature is actually valid. Signature validity must be checked separately by calling validate on the signature info returned by getSignatureInfo.
Note: If you want to check if there is an ink signature for this SignatureFormElement use getOverlappingSignature instead since this method only deals with digital signatures.
Return
true if this signature field is digitally signed, otherwise false.