Add a default value for the --config option

[dshevtsov]

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