Add test for case where ebs volume has no tags.

This commit adds a test for a case where an EBS volume has no tags. When
an EBS volume has no tags, calls to the aws ec2 endpoints `create_volume`
and `describe_volumes` do not include the `Tags` key in the
`response.Volumes[]` object.

However, moto does include the `Tags` key in this case. This discrepancy
in behaviour can result in code passing a moto test but failing in
production.

Sample snippets that trigger this condition:

```
def create_volume_and_then_get_tags_from_response():
    client = boto3.client('ec2', region_name='us-east-1')
    volume_response = client.create_volume(
        Size=10,
        AvailabilityZone='us-east-1a'
    )
    keys = volume_response['Keys']
```

```
def create_volume_and_then_get_tags_from_describe_volumes():
    client = boto3.client('ec2', region_name='us-east-1')
    volume_response = client.create_volume(
        Size=10,
        AvailabilityZone='us-east-1a'
    )
    volume_describe_response = client.describe_volumes()
    keys = volume_describe_response['Volumes'][0]['Keys']
```

Both sample snippets will succeed in a moto test, but fail with a
`KeyError` when using the aws api.

tests/test_ec2/test_elastic_block_store.py

@@ -589,6 +589,18 @@ def test_volume_tag_escaping():
     dict(snaps[0].tags).should.equal({'key': '</closed>'})
 
 
+@mock_ec2
+def test_volume_property_hidden_when_no_tags_exist():
+    ec2_client = boto3.client('ec2', region_name='us-east-1')
+
+    volume_response = ec2_client.create_volume(
+        Size=10,
+        AvailabilityZone='us-east-1a'
+    )
+
+    volume_response.get('Tags').should.equal(None)
+
+
 @freeze_time
 @mock_ec2
 def test_copy_snapshot():