Fix default value handling for multi-value options

The default value for array options will be an array, so it is not suitable to use as the default when processing one of many values for a multi-value option. Using null seems appropriate here, as it indicates the absence of a value and also converts nicely to an empty string (as opposed to an empty array).

Fixes #7689
merged branch jmikola/2.2-fix/gh7689 (PR #7690)

This PR was submitted for the 2.2 branch but it was merged into the 2.1 branch instead (closes #7690).

Discussion
----------

[Console] Fix default value handling for multi-value options

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #7689
| License       | MIT

The default value for array options will be an array, so it is not suitable to use as the default when processing one of many values for a multi-value option. Using null seems appropriate here, as it indicates the absence of a value and also converts nicely to an empty string (as opposed to an empty array).

Commits
-------

a9c28ff [Console] Fix default value handling for multi-value options