Admin: Add validation for multi-URLs with undefined settings in api_get_configuration_value(). Fix setting a default view for the administrative session list on one site of a multi-URL environment could break the list on the other sites

Author: @juan-cortizas-ponte

main/inc/lib/api.lib.php

@@ -9067,6 +9067,9 @@ function api_get_configuration_value($variable)
                 // Try to found element with id = 1 (master portal)
                 if (array_key_exists(1, $_configuration[$variable])) {
                     return $_configuration[$variable][1];
+                } else {
+                    // The value was there for other URLs but not the main URL nor the current URL
+                    return null;
                 }
             }
         }