fix(cues): Ensure track exits before attempting cues access

[sliceofbytes]

Description

Please explain the changes you made here. Commit your changes with npm run commit instead of git commit.

Fix to ensure that the this.ref.nativeElement.track object exists before attempting to access it's cues.

Checklist

Please, check that you have been followed next steps:

• [x] I've read the (contributing)[https://github.com/videogular/videogular2/blob/master/CONTRIBUTING.md] guidelines
• [x] Code compiles correctly (run npm start)
• [x] Created tests (if necessary)
• [x] All tests passing (run npm test)
• [x] Extended the README / documentation (if necessary)

[Elecash]

:tada: This PR is included in version 7.0.2 :tada:

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

Your semantic-release bot :package::rocket: