fix(gatsby): fallback for non-existing config

[thomascorthouts]

This is a proposed way of doing it, I would love to have feedback or a conversation on how to fix/address this in a more Gatsby way.

Description

I found that with changing from Gatsby v2.3.36 to v2.4.2, a Gatsby website would not be able to be served using gatsby serve when there is no gatsby-config.js file in the root folder of the project. This was due to the destructuring on line 72 in the gatsby/src/commands/serve.js file, which threw a TypeError when the command tried to destructure from undefined.

I tried to solve this error by having a fallback to an empty object. But another (and maybe better) approach could also be that it is clearly documented as a needed file in the structure and to add it to the gatsby-starter-hello-world Starter project.

Related Issues

This issue was addressed in #13910

[thomascorthouts]

Follow-up question: Would it make sense to have regression tests whether the starters keep on working after updates?

[m-allanson]

Thanks @thomascorthouts, this is a good fix 👍