* the var will simply be "unknown"
* add a test

The initialization of packer core in JSON also validates that `null` variables were set, except in the case of `packer validate --syntax-only` , but after the refactor to allow to have all commands work with HCL2 and JSON this subtlety was lost.

This refactors the initialisation of the core in order to allow to have `packer validate --syntax-only` not error in case a variable is not set. Since these calls are refactored this works for HCL2 too.

fix #9478

Move vars from Meta to cla args; this forces us to use it in GetConfigFromJSON which in turn forces us into updating console and validate

Co-Authored-By: Adrien Delorme <azr@users.noreply.github.com>

Co-Authored-By: Adrien Delorme <azr@users.noreply.github.com>

borrow wrappedreadline workarounds from terraform and implement a similar check for piped commands; this makes the cli experience much cleaner