fix: signature should be an empty string by default

(would become undefined if the server message was empty)

frappe/public/js/frappe/views/communication.js

@@ -726,7 +726,7 @@ frappe.views.CommunicationComposer = Class.extend({
 
 		if (!signature) {
 			const res = await this.get_default_outgoing_email_account_signature();
-			signature = "<!-- signature-included -->" + res.message.signature;
+			signature = res.message.signature ? "<!-- signature-included -->" + res.message.signature : "";
 		}
 
 		if (signature && !frappe.utils.is_html(signature)) {