Fix event stream test bytes optionality

[JordonPhillips]

This fixes a bug in the event stream test trait where events without bytes would fail to serialize due to not using Optional.ofNullable.

Note that there's no changelog because the trait hasn't been released yet.

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[github-actions]

This pull request does not contain a staged changelog entry. To create one, use the ./.changes/new-change command. For example:

./.changes/new-change --pull-requests "#2825" --type feature --description "Fix event stream test bytes optionality"

Make sure that the description is appropriate for a changelog entry and that the proper feature type is used. See ./.changes/README or run ./.changes/new-change -h for more information.