"Unexpected end of JSON input" error in Diff stage

[nagmesh]

Describe the bug
A clear and concise description of what the bug is.

When pipeline is run, Diff stage fails with

$ /codebuild/output/src1529/src/s3/00/source/packages/@aws-accelerator/accelerator/node_modules/.bin/ts-node --transpile-only cdk.ts --require-approval never diff --config-dir /codebuild/output/src1529/src/s3/01 --partition aws --app cdk.out --stage operations
....
2024-05-30 03:29:22.642 | info | replacements-config | Loading replacements config substitution values
Unexpected end of JSON input
error Command failed with exit code 1.

To Reproduce
Steps to reproduce the behavior.
Have one of the LZA stacks in stuck status like REVIEW_IN_PROGRESS, UPDATE_ROLLBACK_FAILED, etc so that GetTemplate response is empty string

Expected behavior
A clear and concise description of what you expected to happen.
Engine should know the response is non JSON and take an empty JSON for comparison.

Please complete the following information about the solution:

• Version: [e.g. v1.5.1] 1.7.1

To get the version of the solution, you can look at the description of the created AWS CloudFormation stack used to install the LZA (AWSAccelerator-InstallerStack). For example, "(SO0199) Landing Zone Accelerator on AWS. Version 1.5.1.". If the description does not contain the version information, you can look at the Parameters of the stack for the RepositoryBranchName as that should contain the version number.

• Region: [e.g. us-east-1] any region
• Was the solution modified from the version published on this repository? No
• If the answer to the previous question was yes, are the changes available on GitHub?
• Have you checked your service quotas for the services this solution uses? Yes
• Were there any errors in the CloudWatch Logs? Yes, posted above in issue

Screenshots
If applicable, add screenshots to help explain your problem (please DO NOT include sensitive information).

Additional context
Add any other context about the problem here.

When the Landing Zone Accelerator on AWS (LZA) performs a diff operation, it compares the stacks generated by the synthesis process with the existing stacks in the account. To retrieve the existing stacks, the GetTemplate API call is executed. If this call returns an error, an empty JSON response is sent back. In certain cases, stacks may be in a REVIEW_IN_PROGRESS state, which does not generate an error but instead produces a non-JSON output (empty string). When this output is compared, it results in the Unexpected end of JSON input error. Below is an example of what is happening.

> cat tst.yaml
Resources:
  test:
    Type: AWS::CloudFormation::WaitConditionHandle%
> aws cloudformation create-change-set --stack-name test-cg-01 --template-body file://tst.yaml --change-set-name test-cg-01-01 --change-set-type CREATE
{
    "Id": "arn:aws:cloudformation:eu-west-2:redacted:changeSet/test-cg-01-01/3e85e1aa-ac4a-4054-813e-8c3700e45d5d",
    "StackId": "arn:aws:cloudformation:eu-west-2:redacted:stack/test-cg-01/89c18d60-2286-11ef-82b6-0a425d4ba34f"
}
> aws cloudformation describe-stacks --stack-name test-cg-01 --query 'Stacks[0].StackStatus'
"REVIEW_IN_PROGRESS"
> aws cloudformation get-template --stack-name test-cg-01
{
    "TemplateBody": ""
}

If this error persists, it is recommended to check for stacks in the REVIEW_IN_PROGRESS state and delete them. These stacks are not fully created, and deleting them will not cause any service disruption.

[johnraws]

Closing this issue as it is resolved in release/v1.8.0