[FIX] easy_my_coop_ch : validate button must appear when no iban

easy_my_coop_ch/models/coop.py

@@ -23,3 +23,8 @@ class SubscriptionRequest(models.Model):
             req_fields.remove("iban")
 
         return req_fields
+
+    def check_iban(self, iban):
+        if iban:
+            return super(SubscriptionRequest, self).check_iban(iban)
+        return True