Fix to allow author to approve MR

What does this MR do?

Fixes #300218 (closed)

The default setting for Project.merge_requests_author_approval is nil but Ruby evaluates nil? to false causing MR authors to be prevented from approving their own MRs unless the project's "Prevent MR approvals by the author" setting is selected then unselected, which sets it to false.

Either the default value should be false, or the test has to check for nil? as well. This MR does the latter.

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

Related to #300218 (closed)