Allow setting option init without default value

[aapokiiso]

The init method didn't run when configuring an option without a default value, like this:

args
    .options([
        {
            name: 'theme',
            description: 'Build a specific theme in app/design',
            init: val => Array.isArray(val) ? val : [ val ], // Didn't run
        },
        {
            name: 'module',
            description: 'Build a specific module in app/code/local or vendor/',
            defaultValue: [],
            init: val => Array.isArray(val) ? val : [ val ], // Default value defined, runs correctly
        },
        ...
    ]);

I changed the logic slightly so that a default value is not required if you define an init method.

[ntwcklng]

LGTM, thanks!