Fixes #39648 azure_rm_virtualnetwork not handling DNS settings properly on existing virtual networks (#40646)

* check nullable

* add test

* fix

* fix

(cherry picked from commit 7b9864776a)

lib/ansible/modules/cloud/azure/azure_rm_virtualnetwork.py

@@ -257,7 +257,7 @@ class AzureRMVirtualNetwork(AzureRMModuleBase):
                     changed = True
 
                 if self.dns_servers:
-                    existing_dns_set = set(vnet.dhcp_options.dns_servers)
+                    existing_dns_set = set(vnet.dhcp_options.dns_servers) if vnet.dhcp_options else set([])
                     requested_dns_set = set(self.dns_servers)
                     if existing_dns_set != requested_dns_set:
                         self.log('CHANGED: replacing DNS servers')

test/integration/targets/azure_rm_virtualnetwork/tasks/main.yml

@@ -4,6 +4,17 @@
     resource_group: "{{ resource_group }}"
     state: absent
 
+- name: Create virtual network
+  azure_rm_virtualnetwork:
+    name: my_test_network
+    address_prefixes_cidr:
+      - 10.1.0.0/16
+      - 172.100.0.0/16
+    tags:
+      testing: testing
+      delete: on-exit
+    resource_group: "{{ resource_group }}"
+
 - name: Create virtual network
   azure_rm_virtualnetwork:
     name: my_test_network