fix: type-error in email signature code (#18353) (#18363)

* fix: type-error in email signature code

* refactor: import

[skip ci]

Co-authored-by: Ankush Menat <ankush@frappe.io>
(cherry picked from commit 768daf0961)

Co-authored-by: stephen <stephenBDT@users.noreply.github.com>

frappe/core/doctype/communication/communication.py

@@ -19,6 +19,7 @@ from frappe.core.doctype.communication.mixins import CommunicationEmailMixin
 from frappe.core.utils import get_parent_doc
 from frappe.model.document import Document
 from frappe.utils import (
+	cstr,
 	parse_addr,
 	split_emails,
 	strip_html,
@@ -175,7 +176,7 @@ class Communication(Document, CommunicationEmailMixin):
 		if html_signature:
 			_signature = html_signature.renderContents()
 
-		if (_signature or signature) not in self.content:
+		if (cstr(_signature) or signature) not in self.content:
 			self.content = f'{self.content}</p><br><p class="signature">{signature}'
 
 	def before_save(self):