fix: noun function should always return list

[iwong-isp]

The following error gets thrown if the noun function found no errors in the openAPI spec because it currently returns null. This PR modifies the function to always return the list even if empty.

TypeError: Cannot use 'in' operator to search for 'then' in null
    at Object.exports.lintNode (/action/node_modules/@stoplight/spectral/dist/runner/lintNode.js:40:24)
    at callback (/action/node_modules/@stoplight/spectral/dist/runner/runner.js:38:32)
    at JSONPath._handleCallback (/action/node_modules/jsonpath-plus/dist/index-umd.js:626:7)
    at JSONPath._trace (/action/node_modules/jsonpath-plus/dist/index-umd.js:728:12)
    at JSONPath._trace (/action/node_modules/jsonpath-plus/dist/index-umd.js:689:19)
    at /action/node_modules/jsonpath-plus/dist/index-umd.js:693:21
    at /action/node_modules/jsonpath-plus/dist/index-umd.js:909:9
    at Array.forEach (<anonymous>)
    at JSONPath._walk (/action/node_modules/jsonpath-plus/dist/index-umd.js:908:24)
    at JSONPath._trace (/action/node_modules/jsonpath-plus/dist/index-umd.js:692:12)