Set internal default property value to undefined instead of null & add PostgreSQL uuid support

[dxg]

• Set internal default property value to undefined instead of null.

  • This will prevent null values being explicitly sent to the database when no value was assigned and instead result in the database setting the column to null, or generating a default value.
  • Properties with an internal value of undefined will still return null when accessed.
  • Setting a previously filled property to undefined will still set it to null in the DB.
  • No ORM tests were broken by this change, and as such, the impact of this should be limited ot a very small number of corner cases.

• Add PostgreSQL uuid column support.

• Allow specifying defaultExpression (eg. uuid_generate_v4() for PostgreSQL or uuid() for MySQL) to be executed by the database engine for generating default values.