fix(FEC-12527): turning on hls debug mode in config causes the player to crash

[SivanA-Kaltura]

Description of the Changes

In some cases hls.js triggers our event handlers (specifically _onAudioTrackSwitched) before the _onManifestLoaded handler. When _onAudioTracksSwitched calls _playerTracks.find, which is undefined, it crashes.
The solution is to initialize _playerTracks with an empty array before _onManifestLoaded fills it with the real values.

CheckLists

• changes have been done against master branch, and PR does not conflict
• new unit / functional tests have been added (whenever applicable)
• test are passing in local environment
• Travis tests are passing (or test results are not worse than on master branch :))
• Docs have been updated