Added null handling in scanner.ts

[ashwinGokhale]

Details

Null pointer exception when reading the scan results:

##[error][Exception]ErrorWithCause: An error occurred while scanning website page http://localhost:9009/ at Logger.trackExceptionAny (/__w/_tasks/accessibility-insights_4811a442-2fd3-5aa8-ba1a-14cb7e24c113/2.0.6/index.js:189664:29) at Scanner. (/__w/_tasks/accessibility-insights_4811a442-2fd3-5aa8-ba1a-14cb7e24c113/2.0.6/index.js:190823:29) at Generator.next () at fulfilled (/__w/_tasks/accessibility-insights_4811a442-2fd3-5aa8-ba1a-14cb7e24c113/2.0.6/index.js:190741:58) caused by: TypeError: Cannot read property 'baseUrl' of undefined at Scanner.getCombinedReportParameters (/__w/_tasks/accessibility-insights_4811a442-2fd3-5aa8-ba1a-14cb7e24c113/2.0.6/index.js:190837:60) at Scanner. (/__w/_tasks/accessibility-insights_4811a442-2fd3-5aa8-ba1a-14cb7e24c113/2.0.6/index.js:190815:55) at Generator.next () at fulfilled (/__w/_tasks/accessibility-insights_4811a442-2fd3-5aa8-ba1a-14cb7e24c113/2.0.6/index.js:190741:58) Accessibility scanning of URL http://localhost:9009/ completed

Motivation

Addresses #2014

Context

Pull request checklist

• [x] Addresses an existing issue: Fixes #0000
• [x] Added relevant unit test for your changes. (yarn test)
• [x] Verified code coverage for the changes made. Check coverage report at: <rootDir>/test-results/unit/coverage
• [x] Ran precheckin (yarn precheckin)

[v-viyada]

Thanks for submitting PR. We are interested in review this PR but we cannot take this immediately due to some other pending priority issues. We will take this later.