system_site migrates default 403, 404 and front page paths as forward slash (/)

Problem/Motivation

The system_site migration migrates empty (or missing) site_403 and site_404 variables as front page path (/).
In addition to this, when drupal_weight_select_max is unavailable in the source DB, it will be set to 0, but in these cases, we should migrate its default value, which is 100.

h2. Steps to Reproduce
Execute system_site migration with empty (or missing) site_403 and site_404 variables.

h2. Expected Result

• Empty or missing site_403 variable is migrated as an empty 403 page config.
• Empty or missing site_404 variable is migrated as an empty 404 page config.
• Missing drupal_weight_select_max is migrated as weight_select_max set to 100.

Steps to reproduce

Execute system_site migration with empty (or missing) site_403 and site_404 variables.

Proposed resolution

Skip processing of 'page/403' and 'page/404' destination properties if the corresponding Drupal 6 / Drupal 7 variables are empty.

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet