Enable 'Drupal.Commenting.VariableComment.MissingVar' coding standard

Problem/Motivation

See #2909372-14: [PP-1] Enable 'Drupal.Commenting.VariableComment.MissingVar' coding standard for the reason why this child issue was created and #2909364: [meta] Fix 'Drupal.Commenting.VariableComment' coding standard for general instructions about working on Drupal.Commenting.VariableComment coding standard errors.

Steps to reproduce

Proposed resolution

Fix errors found by Drupal.Commenting.VariableComment.MissingVar for parameters that do not have a default value.

Remaining tasks

MR 3689 is for 11.x
Review
Commit

User interface changes

API changes

Data model changes

Release notes snippet