cloudstack: fix state=absent, ip_address not None but falsy (#3483)

lib/ansible/modules/extras/cloud/cloudstack/cs_ip_address.py

@@ -208,8 +208,8 @@ class AnsibleCloudStackIPAddress(AnsibleCloudStack):
 
     def disassociate_ip_address(self):
         ip_address = self.get_ip_address()
-        if ip_address is None:
+        if not ip_address:
-            return ip_address
+            return None
         if ip_address['isstaticnat']:
             self.module.fail_json(msg="IP address is allocated via static nat")