Monorepo flow: When validating a release spec, ensure list of packages is non-empty

Add a check to [`validateReleaseSpecification`](https://github.com/MetaMask/create-release-branch/blob/68072dac631087984b8d7f59df21ffa4407de35c/src/release-specification.ts#L164) which will check whether the list of packages is empty; if it is, then throw an error.

Start in src/release-specification.ts at validateReleaseSpecification, linked from the issue. Read how the release specification and package list are currently validated, then add the requested empty-list rejection. Done means a release specification with no packages produces an error while non-empty package lists continue to validate.

Written by the indexing model from the issue text.