Fix the Host.display_name default value

[Glutexo]

If a Host instance is created without passing a display_name value, it becomes unsaveable. The reason is the default value for the property. Instead of None, the column object is assigned, which is obviously a bug.

To reproduce:

def create_host():
    insights_id = uuid4()
    canonical_facts = {"insights_id": str(insights_id)}
    return Host(canonical_facts, account="abc123")

if __name__ == "__main__":
    application = create_app("cli")
    with application.app_context():
        host = create_host()
        db.session.add(host)
        db.session.commit()

💥

TypeError: Boolean value of this clause is not defined