Add default value for merge_requests_author_approval

What does this MR do?

merge_requests_author_approval is a legacy boolean attribute that allows nil values (and thus the ever wonderful tri-state boolean situation...)

This MR seeks to correct this by

• adding a default value of false
• handling nil in attribute accessors
• adding a NOT NULL constraint via a post migration

Docs on adding NOT NULL constraints

I don't believe we need to update existing records, since in this case we can modify/update existing accessor methods.

Related to #324087 (closed)

Migrations

Up

== 20210517221612 AddDefaultValueToMergeRequestsAuthorApprovalOnProjects: migrating
-- change_column_default(:projects, :merge_requests_author_approval, false)
   -> 0.0075s
== 20210517221612 AddDefaultValueToMergeRequestsAuthorApprovalOnProjects: migrated (0.0116s)

Down

== 20210517221612 AddDefaultValueToMergeRequestsAuthorApprovalOnProjects: reverting
-- change_column_default(:projects, :merge_requests_author_approval, nil)
   -> 0.0035s
== 20210517221612 AddDefaultValueToMergeRequestsAuthorApprovalOnProjects: reverted (0.0112s)