fix: populate empty SNIs with an empty array

Kong returns back an empty JSON array instead of a `null` when the
SNI array is empty:

```
{
    "data": [
        {
            "cert": "-truncated-",
            "created_at": 1584219121,
            "id": "3e83146b-7139-4306-aa0c-f95ba3a9e315",
            "key": "-truncated-",
            "snis": [],
            "tags": null
        }
    ],
    "next": null
}
```

Kong deviates from it's regular behavior of returning a `null`.
This causes decK to detect a diff an existing and desired certificate
because the two certificates have different empty SNIs, one has an empty
array ([]*string{}) while the other is `nil`.

This commit populates a nil SNIs field with an empty array.

This bug only shows up when there are no SNIs associates with a
certificate in Kong.

Fix #131
From #132

file/builder.go

@@ -83,6 +83,9 @@ func (b *stateBuilder) certificates() {
 			}
 		}
 		utils.MustMergeTags(&c.Certificate, b.selectTags)
+		if c.Certificate.SNIs == nil {
+			c.Certificate.SNIs = []*string{}
+		}
 
 		b.rawState.Certificates = append(b.rawState.Certificates,
 			&c.Certificate)

file/builder_test.go

@@ -1175,6 +1175,7 @@ func Test_stateBuilder_certificates(t *testing.T) {
 						ID:   kong.String("538c7f96-b164-4f1b-97bb-9f4bb472e89f"),
 						Cert: kong.String("foo"),
 						Key:  kong.String("bar"),
+						SNIs: []*string{},
 					},
 				},
 			},
@@ -1200,6 +1201,7 @@ func Test_stateBuilder_certificates(t *testing.T) {
 						ID:   kong.String("4bfcb11f-c962-4817-83e5-9433cf20b663"),
 						Cert: kong.String("foo"),
 						Key:  kong.String("bar"),
+						SNIs: []*string{},
 					},
 				},
 			},