fix: prevent current time display showing 0:00 during seek

[mister-ben]

Fixes #9133. #8988 causes the current time display to show 0:00 while seeking as it's getting the current time from a method on seek bar that won't always return a useful value.

Adds a null check to allow fall through to getting currentTime.

Requirements Checklist

• Feature implemented / Bug fixed
• If necessary, more likely in a feature request than a bug fix
  • Change has been verified in an actual browser (Chrome, Firefox, IE)
  • Unit Tests updated or fixed
  • Has no DOM changes which impact accessiblilty or trigger warnings (e.g. Chrome issues tab)
  • Has no changes to JSDoc which cause npm run docs:api to error
• Reviewed by Two Core Contributors

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 84.31%. Comparing base (d203eab) to head (ecf2567).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #9135   +/-   ##
=======================================
  Coverage   84.31%   84.31%           
=======================================
  Files         120      120           
  Lines        8154     8154           
  Branches     1964     1964           
=======================================
  Hits         6875     6875           
  Misses       1279     1279           

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