With Ruby 2.6 using `**kwargs` here will not work because it would be an
empty hash that we then expect to be submitted to
`Gitlab::Tracking#event`.

An example when using `expect_snowplow_event`:

```
expect_snowplow_event(
  category: 'cluster:services:prometheus',
  action: 'enabled_manual_prometheus'
)
```

Error:
```
Gitlab::Tracking received :event with unexpected arguments
expected:
  ("cluster:services:prometheus", "enabled_manual_prometheus", {})
got:
  ("cluster:services:prometheus", "disabled_manual_prometheus") (1 time)
("cluster:services:prometheus", "enabled_manual_prometheus") (1 time)
```

This has not been an issue so far because the other specs we migrated
always provided additional `kwargs`.

To prevent this, we only expect the `**kwargs` parameter when it exists.
However, this will get fixed with Ruby 2.7 and we can remove the
condition again.