TypeError issue reported by OAD in CI for azure-rest-api-specs repo

@sergey-shandar
https://travis-ci.org/Azure/azure-rest-api-specs/jobs/513289560
```
TypeError [ERR_INVALID_ARG_TYPE]: The "path" argument must be of type string. Received type undefined

at validateString (internal/validators.js:125:11)

at Object.join (path.js:1147:7)

at exports.Logger.get [as filepath] (/home/travis/build/Azure/azure-rest-api-specs/node_modules/@azure/oad/dist/lib/util/logging.js:92:39)

at Object.compare (/home/travis/build/Azure/azure-rest-api-specs/node_modules/@azure/oad/dist/lib/validate.js:26:67)

at runOad (/home/travis/build/Azure/azure-rest-api-specs/scripts/breaking-change.js:68:26)

at runScript (/home/travis/build/Azure/azure-rest-api-specs/scripts/breaking-change.js:148:27)

at process._tickCallback (internal/process/next_tick.js:68:7)
```

Start with the Travis CI job linked in the issue and inspect scripts/breaking-change.js around runOad at line 68 and its caller around line 148. Trace how the OAD logger reaches logging.js and validate.js, then reproduce the failure in the CI workflow. Done means the breaking-change check no longer throws the undefined path TypeError.

索引モデルが issue の本文から書いたものです。