Make sure epic_id param is handled properly

What does this MR do?

It fixes a problem with assigning an epic to an issue during issue creation and using quick actions.

When quick action is used for assigning an epic to an issue that is just being created, both epic and epic_id params are used in Issues::CreateService. epic_id is empty in this case.

In the service we have epic = epic_param || find_epic(issue, epic_id) - when there is epic param, it never goes to find_epic which then deletes the epic_id key and set epic_param based on it.

This MR fixes this problem.

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Related issue #245026 (closed)