fix eip physical_resource_id not returning an public_ip if it is in EC2 classic.

moto/ec2/models.py

@@ -1964,7 +1964,7 @@ class ElasticAddress(object):
 
     @property
     def physical_resource_id(self):
-        return self.allocation_id
+        return self.allocation_id if self.allocation_id else self.public_ip
 
     def get_cfn_attribute(self, attribute_name):
         from moto.cloudformation.exceptions import UnformattedGetAttTemplateException