[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).