fix: Remove img el when there's no poster source

[mister-ben]

Description

If there is no poster source, an img el can't be present as it would be invalid.

Specific Changes proposed

Use a div (again) as the top level element of the PosterImage, to keep position in the DOM. Insert/remove the picture/img els when a poster source is set/unset.
Fixes #8122

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
  • Docs/guides updated
  • Example created (starter template on JSBin)
• Reviewed by Two Core Contributors

[codecov]

Codecov Report

Merging #8130 (4580437) into main (b306ce6) will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #8130      +/-   ##
==========================================
+ Coverage   82.00%   82.02%   +0.02%     
==========================================
  Files         110      110              
  Lines        7339     7344       +5     
  Branches     1770     1773       +3     
==========================================
+ Hits         6018     6024       +6     
+ Misses       1321     1320       -1     

Impacted Files	Coverage Δ
src/js/poster-image.js	78.26% <100.00%> (+2.65%)	⬆️
src/js/tracks/text-track.js	94.87% <0.00%> (+0.64%)	⬆️

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