check audio textTracks before trying to use them for #1871

[wgilling]

GitHub Issue: (Islandora/documentation#1871)

What does this Pull Request do?

This should fix the error that was noticed with issue #1871. The adjusted code only checks that the textTracks exist before trying to loop through the cues within them to set the HTML accordingly.

How should this be tested?

To see the problem that is observed in the related issue 1871:

• The Format for the audio file must also be set to "Audio with Captions" for either Source or Default http://localhost:8000/admin/structure/media/manage/audio/display or http://localhost:8000/admin/structure/media/manage/audio/display/source... otherwise the audio.js file is not attached via the twig template.
• With an item that has an audio model -- and media attached that is a simple MP3 file or WAV file (for example, https://file-examples-com.github.io/uploads/2017/11/file_example_MP3_700KB.mp3 can be downloaded locally and used as an item's media) -- and there has not been a .vtt file associated with the media, then the error would be visible on the items' media page in the (Chrome | Inspect) javascript console.
• confirm that the audio that DOES have tracks works as expected

To test the pull request:

1. pull in the code change for this PR
2. Check the item that exhibited the javascript console error as above and confirm that the error message is gone.

[wgilling]

updated PR to add additional configuration required to see this issue.

[ajstanley]

Works exactly as promised.
Tested on an ISLE build.