Fix Snowplow spec helper for Ruby 2.6.6

What does this MR do?

@splattael experienced issues with the Snowplow Spec helper in #259282 (comment 425243784) with Ruby 2.6. Thanks @syasonik for the pragmatic idea how to fix it for now 🙏

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", "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.

See https://www.ruby-lang.org/en/news/2019/12/12/separation-of-positional-and-keyword-arguments-in-ruby-3-0/ "Double splat with an empty hash (**{}) passes no arguments" for more information.

To revert the change again after migrating to Ruby 2.7, there is #263430 (closed)

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