make pClientAmiId parameter optional as per the docs

[vlas-sokolov]

The docs suggest that two subnet parameters and one VPC are enough for the quick start of the template. However, when I create a stack from cloudhsm.yml template file, with the following parameters:

[
  {
    "ParameterKey": "pSubnets",
    "ParameterValue": "subnet-00000000000000000"
  },
  {
    "ParameterKey": "pClientVpcId",
    "ParameterValue": "vpc-00000000000000000"
  },
  {
    "ParameterKey": "pClientSubnet",
    "ParameterValue": "subnet-00000000000000000"
  }
]

, I'm getting the following error An error occurred (ValidationError) when calling the CreateStack operation: Parameters: [pClientAmiId] must have values. It appears that the pClientAmiId is the only parameter that is marked as optional but doesn't get its own default value. This PR adds it to be a blank string as line 147 seems to require. I deployed the stack afterwards and it seems to proceed normally for a while (for proper deployment I did have to set a pClientAmiId value but that's because the region I use doesn't have t3a instance family so I had to set manual values for type and ami).

[vlas-sokolov]

Maybe should add that the stack deployment command I used was
aws cloudformation create-stack --template-url https://some-private-bucket-uri/cloudhsm.yml --stack-name CloudHSMStack --parameters file://cfn_params.json --capabilities CAPABILITY_NAMED_IAM, where cfn_params.json file contents were the parameters I pasted in the first message.

[Roedy13]

+1 on this