Add a default value for the --config option

### Expected Behaviour

When I run `aio changelog:generate`, the tool gets the `config.json` file for `--config`.

### Actual Behaviour

When I run `aio changelog:generate`, I see `TypeError: Cannot read property 'forEach' of undefined`.

### Reproduce Scenario (including but not limited to)

#### Steps to Reproduce

Run `aio changelog:generate`.

#### Platform and Version

macOS 11.4

#### Sample Code that illustrates the problem

#### Logs taken while reproducing problem

```
$ aio changelog:generate
› Warning: @adobe/aio-cli update available from 7.0.0 to 7.1.0.
› Run npm install -g @adobe/aio-cli to update.

==========================
Start changelog generation
==========================
... !
TypeError: Cannot read property 'forEach' of undefined
```

Start at the `aio changelog:generate` command entry point and inspect how the `--config` option is handled when omitted. Use the reported command and error as the reproduction case; done means the command uses `config.json` by default and no longer raises the `forEach` TypeError.

Written by the indexing model from the issue text.