Do not try to encode metadata if it is None

[thpica]

SUMMARY

In the gcp_compute_instance module, when not specifying the metadata key or leaving it undefined, it crashes trying to iterate on a None here: https://github.com/ansible/ansible/blob/devel/lib/ansible/modules/cloud/google/gcp_compute_instance.py#L1138

This PR addresses it by skipping the call to metadata_encoder() if request['metadata'] is None.

ISSUE TYPE

• Bugfix Pull Request

COMPONENT NAME

gcp_compute_instance

ANSIBLE VERSION

ansible 2.7.0.post0
  config file = /etc/ansible/ansible.cfg
  configured module search path = [u'/home/tpicariello/.ansible/plugins/modules', u'/usr/share/ansible/plugins/modules']
  ansible python module location = /home/tpicariello/.pyenv/batcheval/local/lib/python2.7/site-packages/ansible
  executable location = /home/tpicariello/.pyenv/project/bin/ansible
  python version = 2.7.12 (default, Jul 18 2016, 15:02:52) [GCC 4.8.4]

ADDITIONAL INFORMATION

Omitting the metadata key in the example given by the doc allows to reproduce the issue:

- name: create a instance
  gcp_compute_instance:
      name: "test_object"
      machine_type: n1-standard-1
      disks:
      - auto_delete: true
        boot: true
        source: "{{ disk }}"
      network_interfaces:
      - network: "{{ network }}"
        access_configs:
        - name: External NAT
          nat_ip: "{{ address }}"
          type: ONE_TO_ONE_NAT
      zone: us-central1-a
      project: "test_project"
      auth_kind: "service_account"
      service_account_file: "/tmp/auth.pem"
      state: present

Reopening #46491 on devel

[mkrizek]

This looks good. Please add a changelog fragment as mentioned in #46491 (comment). Thanks!

[ansibot]

@thpica: thank you for submitting this pull-request!

cc @erjohnso @rambleraptor
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