Add a constraint to the component table in the database to prevent the lead column from being empty

Issue description

There can be some situation where a component is created with an empty lead (especially while using a 3rd party add-on).
In this case, if an XML backup is created and used to perform a project import, the project import will fail with the following error:

There are '1' required user(s) that JIRA can not automatically create.

Suggested solution to fix this behavior

The suggestion would be to:

• either add a constraint to the component table in the database to prevent the lead column from being empty.
• or modify the ProjectComponentManager.create() to stop the creation of the component if the lead field is empty (OR if the lead field is empty, replace this field with a null field)