warn about assign_public_ip immutability only if explicitly set (#37974)

Change assign_public_ip to default to None rather than False so
that we can detect whether the value is being explicitly set or
not, and only warn if it is explicitly set to False for something
with a public_dns_name

Fixes #37985

lib/ansible/modules/cloud/amazon/ec2.py

@@ -134,7 +134,6 @@ options:
     description:
       - when provisioning within vpc, assign a public IP address. Boto library must be 2.13.0+
     type: bool
-    default: 'no'
   private_ip:
     version_added: "1.2"
     description:
@@ -1560,7 +1559,7 @@ def warn_if_public_ip_assignment_changed(module, instance):
 
     # Check that public ip assignment is the same and warn if not
     public_dns_name = getattr(instance, 'public_dns_name', None)
-    if (assign_public_ip or public_dns_name) and (not public_dns_name or not assign_public_ip):
+    if (assign_public_ip or public_dns_name) and (not public_dns_name or assign_public_ip is False):
         module.warn("Unable to modify public ip assignment to {0} for instance {1}. "
                     "Whether or not to assign a public IP is determined during instance creation.".format(assign_public_ip, instance.id))
 
@@ -1590,7 +1589,7 @@ def main():
             user_data=dict(),
             instance_tags=dict(type='dict'),
             vpc_subnet_id=dict(),
-            assign_public_ip=dict(type='bool', default=False),
+            assign_public_ip=dict(type='bool'),
             private_ip=dict(),
             instance_profile_name=dict(),
             instance_ids=dict(type='list', aliases=['instance_id']),