fix(router): fix return value for getTransition of NSRouterLink

[flore2003]

Return undefined transition value from getTransition method of NSRouterLink when boolean value is used for pageTransition input. Fixes #1784

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-angular/blob/master/DevelopmentWorkflow.md#running-the-tests
• Tests for the changes are included.

What is the current behavior?

The getTransition method of NSRouterLink returns an empty object value for the transition property, when the ´pageTransition´ input is given a boolean value. This leads to the defaultTransition of the Frame not being applied.

What is the new behavior?

The getTransition method of NSRouterLink now returns undefined as the value for the transition property.

Fixes #1784.

[flore2003]

Any news on this PR?

[VladimirAmiorkov]

Hi @flore2003 ,

The PR has been reviewed but is not yet merged because we are in the process of a release which is planned for later this week. After that release we will update this PR to the latest branch and process it. Sorry for the delay caused by our current release plan.