Set author on issuable to current user if it is not already set

What does this MR do and why?

Set current user as issuable author if the issuable author is not already set. This is needed sometimes when we have used a custom issuable Builder to build the issuable and we are calling the create service with the given built issuable to be created. E.g. import epic scenario.

• Tracking issue: User mapping - Epic and EpicIssue work item (#478819 - closed)
• MR with original fix in master: Set author on issuable to current user if it is... (!168558 - merged)

MR acceptance checklist

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• This MR is backporting a bug fix, documentation update, or spec fix, previously merged in the default branch.
• The MR that fixed the bug on the default branch has been deployed to GitLab.com (not applicable for documentation or spec changes).
• This MR has a severity label assigned (if applicable).
• Set the milestone of the merge request to match the target backport branch version.
• This MR has been approved by a maintainer (only one approval is required).
• Ensure the e2e:test-on-omnibus-ee job has either succeeded or been approved by a Software Engineer in Test.

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