<!--- Verify first that your issue is not already reported on GitHub -->
<!--- Also test if the latest release and devel branch are affected too -->
<!--- Complete *all* sections as described, this form is processed automatically -->

##### SUMMARY

The gcp dynamic inventory fails.

##### ISSUE TYPE
- Bug Report

##### COMPONENT NAME
- `gcp_compute`

##### ANSIBLE VERSION
<!--- Paste verbatim output from "ansible --version" between quotes -->
```paste below
ansible 2.8.3
  config file = /home/user/repos/project/ansible.cfg
  configured module search path = ['/home/user/.ansible/plugins/modules', '/usr/share/ansible/plugins/modules']
  ansible python module location = /home/user/.local/share/virtualenvs/project-ymgbqjq6/lib/python3.7/site-packages/ansible
  executable location = /home/user/.local/share/virtualenvs/project-ymgbqjq6/bin/ansible
  python version = 3.7.4 (default, Jul  9 2019, 03:52:42) [GCC 5.4.0 20160609]
```

##### CONFIGURATION
<!--- Paste verbatim output from "ansible-config dump --only-changed" between quotes -->
```paste below
ANSIBLE_NOCOWS(/home/user/repos/project/ansible.cfg) = True
DEFAULT_ROLES_PATH(/home/user/repos/project/ansible.cfg) = ['/home/user/repos/project/roles']
DEFAULT_VAULT_PASSWORD_FILE(/home/user/repos/project/ansible.cfg) = /home/user/.secret/secret.txt
INVENTORY_ENABLED(/home/user/repos/project/ansible.cfg) = ['gcp_compute']
```

##### OS / ENVIRONMENT
<!--- Provide all relevant information below, e.g. target OS versions, network device firmware, etc. -->
- Ubuntu 16.04

##### STEPS TO REPRODUCE
<!--- Describe exactly how to reproduce the problem, using a minimal test-case -->

Enable `gcp_compute` plugin in the inventory section of `ansible.cfg`:

```ini
[inventory]
enable_plugins = gcp_compute
```

Make `inventory.gcp.yml` file:

```yaml
plugin: gcp_compute
projects:
  - gcp-test-project-1245
auth_kind: serviceaccount
service_account_file: "~/.secret/gcp-test-project-1245.json"
```
Try to list inventories:

```sh
ansible-inventory --list -i inventory.gcp.yml
```

<!--- HINT: You can paste gist.github.com links for larger files -->

##### EXPECTED RESULTS
<!--- Describe what you expected to happen when running the steps above -->
JSON list of inventories.

##### ACTUAL RESULTS
<!--- Describe what actually happened. If possible run with extra verbosity (-vvvv) -->

<!--- Paste verbatim command output between quotes -->
```paste below
ansible-inventory 2.8.3
  config file = /home/user/repos/project/ansible.cfg
  configured module search path = ['/home/user/.ansible/plugins/modules', '/usr/share/ansible/plugins/modules']
  ansible python module location = /home/user/.local/share/virtualenvs/project-ymgbqjq6/lib/python3.7/site-packages/ansible
  executable location = /home/user/.local/share/virtualenvs/project-ymgbqjq6/bin/ansible-inventory
  python version = 3.7.4 (default, Jul  9 2019, 03:52:42) [GCC 5.4.0 20160609]
Using /home/user/repos/project/ansible.cfg as config file
setting up inventory plugins
 [WARNING]:  * Failed to parse /home/user/repos/project/inventory.gcp.yml with gcp_compute plugin: 'NoneType' object has no attribute 'get'

  File "/home/user/.local/share/virtualenvs/project-ymgbqjq6/lib/python3.7/site-packages/ansible/inventory/manager.py", line 268, in parse_source
    plugin.parse(self._inventory, self._loader, source, cache=cache)
  File "/home/user/.local/share/virtualenvs/project-ymgbqjq6/lib/python3.7/site-packages/ansible/plugins/inventory/gcp_compute.py", line 485, in parse
    for key, value in resp.get('items').items():

 [WARNING]: Unable to parse /home/user/repos/project/inventory.gcp.yml as an inventory source

 [WARNING]: No inventory was parsed, only implicit localhost is available

{
    "_meta": {
        "hostvars": {}
    },
    "all": {
        "children": [
            "ungrouped"
        ]
    }
}
```

'''
            :param params: a dict containing all of the fields relevant to build URL
            :param link: a formatted URL
            :param query: a formatted query string
            :return the JSON response containing a list of instances.
        '''
        response = self.auth_session.get(link, params={'filter': query})
        return self._return_if_object(self.fake_module, response)
    def _get_query_options(self, filters):
        '''
            :param config_data: contents of the inventory config file
            :return A fully built query string
        '''
        if not filters:
            return ''
        if len(filters) == 1:
            return filters[0]
        else:
            queries = []
            for f in filters:
                
                if f[0] != '(' and f[-1] != ')':
                    queries.append("(%s)" % ''.join(f))
                else:
                    queries.append(f)
            return ' '.join(queries)
    def _return_if_object(self, module, response):