Normalize empty params in ActionCable logging

What does this MR do?

Related to gitlab-com/gl-infra/delivery#1210 (comment 414925769)

Lograge's ActionCable subscriber sets the method, path, and format to empty hashes in https://github.com/roidrage/lograge/blob/v0.11.2/lib/lograge/log_subscribers/action_cable.rb#L14-L16.

We transform these to empty strings so that the data type will be consistent.

Screenshots

Does this MR meet the acceptance criteria?

Conformity

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