fix: migrate fails when karma.config.js is missing

[rosen-vladimirov]

In case you've installed nativescript-unit-test-runner , but you've not used tns test init, tns migrate command fails with error:
Could not migrate the project! The error is: Error: ENOENT: no such file or directory, open '<project dir>/.migration_backup/karma.conf.js'
Skip the migration of the karma.conf.js file in case it is missing. Also, ensure karma-webpack is always added by the migration when nativescript-unit-test-runner is migrated.

PR Checklist

• The PR title follows our guidelines: https://github.com/NativeScript/NativeScript/blob/master/CONTRIBUTING.md#commit-messages.
• There is an issue for the bug/feature this PR is for. To avoid wasting your time, it's best to open a suggestion issue first and wait for approval before working on it.
• You have signed the CLA.
• All existing tests are passing: https://github.com/NativeScript/nativescript-cli/blob/master/CONTRIBUTING.md#contribute-to-the-code-base
• Tests for the changes are included.

What is the current behavior?

When you execute the following:

$ tns create myApp --js
$ cd myApp
$ npm i --save-dev nativescript-unit-test-runner
$ tns migrate

The command fails with error: Could not migrate the project! The error is: Error: ENOENT: no such file or directory, open '<project dir>/.migration_backup/karma.conf.js'

What is the new behavior?

Command successfully migrates project which has nativescript-unit-test-runner, but does not have karma.conf.js file.

Fixes/Implements/Closes #[Issue Number].