AV: Add empty img alt tag to placeholder

[HarveyPeachey]

Resolves #1840

Overall change: Updated image alt to be empty to follow desired behaviour for screen reader

Code changes:

• Replaced alt="Image Alt" with alt="" on video image placeholder
• Updated snapshots

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• I have assigned myself to this PR and the corresponding issues
• Automated jest tests added (for new features) or updated (for existing features)
• This PR requires manual testing

[HarveyPeachey]

Requires a11y swarm: Testing on Storybook Psammead MediaPlayer

A11y Swarm
https://bbc.github.io/accessibility-news-and-you/accessibility-news-and-testers

Screen Readers (IE11):

• ZoomText Magnifier/Reader - 8, 9, 10 - MouseHover, Speakit, AppRdr tool doesn't say anything when placeholder image is clicked only when the play button is clicked.
• Dragon NaturallySpeaking
• JAWS - Doesn't read out any alt on the image skips straight to play button.
• Read&Write - N/A

Mac, iOS, and Android:

• VoiceOver iOS (iPhone/iPad) - Doesn't read out any alt on the image skips straight to play button.
• VoiceOver OS (Mac) - Doesn't read out any alt on the image skips straight to play button.
• TalkBack - Ray (Chrome on Android)

[jamesdonoh]

Something nice and easy for a change :-)