Do not try to encode metadata if it is None (#46739)

* Do not try to encode metadata if it is None

* Add changelog fragment

* Fix fragment missing EOF line

changelogs/fragments/46739-gcp-compute-instance-metadata.yaml

@@ -0,0 +1,2 @@
+bugfixes:
+  - "gcp_compute_instance - fix crash when the instance metadata is not set"

lib/ansible/modules/cloud/google/gcp_compute_instance.py

@@ -1106,7 +1106,7 @@ def raise_if_errors(response, err_path, module):
 
 
 def encode_request(request, module):
-    if 'metadata' in request:
+    if 'metadata' in request and request['metadata'] is not None:
         request['metadata'] = metadata_encoder(request['metadata'])
     return request