Use to_json for matrix_mautrix_whatsapp_extev_polls

roles/custom/matrix-bridge-mautrix-whatsapp/templates/config.yaml.j2

@@ -292,7 +292,7 @@ bridge:
     # Send galleries as a single event? This is not an MSC (yet).
     beeper_galleries: false
     # Should polls be sent using MSC3381 event types?
-    extev_polls: {{ matrix_mautrix_whatsapp_extev_polls }}
+    extev_polls: {{ matrix_mautrix_whatsapp_extev_polls | to_json }}
     # Should cross-chat replies from WhatsApp be bridged? Most servers and clients don't support this.
     cross_room_replies: false
     # Disable generating reply fallbacks? Some extremely bad clients still rely on them,