Setting null value on optional resources

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#### Overview of the Issue

I am trying to set null values on optional fields ( org_arns) and I am getting an error saying that null cannot be set for this value. I am trying to set this to null due to the fact when testing our branches we do not want to send the ami to the org rather to a single account for validation and testing. Once that is complete we would then send it to the org where every account would receive it.

#### Reproduction Steps

set value ami_org_arns in variables file to default = null

### Plugin and Packer version

From `packer version` 1.2.6/1.2.7

### Operating system and Environment details

Rhel8, win2019

### Log Fragments and crash.log files

Error: null value is not allowed: [{{} ami_org_arns} {{} {{{{} 78}} 0xc000127bc0}}]