fix: use empty string to remove server from its placement group (ansi…

…ble-collections#489)

##### SUMMARY
- Do not remove the server from its placement group when the
`placement_group` argument is not specified.
- Pass an empty string to the `placement_group` argument to remove a
server from its placement group.

changelogs/fragments/server-empty-string-arguments.yml

@@ -0,0 +1,3 @@
+bugfixes:
+  - server - Do not remove the server from its placement group when the `placement_group` argument is not specified.
+  - server - Pass an empty string to the `placement_group` argument to remove a server from its placement group.

tests/integration/targets/placement_group/tasks/test.yml

@@ -80,7 +80,7 @@
 - name: test remove server from placement group
   hetzner.hcloud.server:
     name: "{{ hcloud_server_name }}"
-    placement_group: null
+    placement_group: ""
     state: present
   register: result
 - name: verify remove server from placement group