fix: Check if header/footer html exists before trying to extract

- issue: AttributeError: 'NoneType' object has no attribute 'extract', when no letterhead html was present

frappe/utils/pdf.py

@@ -189,7 +189,8 @@ def prepare_header_footer(soup: BeautifulSoup):
 
 	# extract header and footer
 	for html_id in ("header-html", "footer-html"):
-		if content := soup.find(id=html_id).extract():
+		if content := soup.find(id=html_id):
+			content = content.extract()
 			# `header/footer-html` are extracted, rendered as html
 			# and passed in wkhtmltopdf options (as '--header/footer-html')
 			# Remove instances of them from main content for render_template