[ARM Incremental TypeSpec] Return false if swagger cannot be parsed as JSON

[mikeharder]

Fixes this error, or any error parsing swagger that's not valid JSON:

execRoot("git show  HEAD:specification/appconfiguration/resource-manager/Microsoft.AppConfiguration/stable/2024-06-01/appconfiguration.json")
SyntaxError: Expected ',' or '}' after property value in JSON at position 56912
    at JSON.parse (<anonymous>)
    at incrementalTypeSpec (file:///home/runner/work/azure-rest-api-specs/azure-rest-api-specs/.github/workflows/src/arm-incremental-typespec-preview.js:30:29)

https://github.com/Azure/azure-rest-api-specs/actions/runs/13528838065/job/37805953795#step:3:58

[openapi-pipeline-app]

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