[Technical] Open stickers using the thumbnail url if the main url is empty

[surakin]

This is a workaround for matrix-org/matrix-spec#1667

Several bridges (see mautrix/whatsapp#680) correctly send stickers as encrypted images but the spec doesn't cover that and the Rust SDK fails to parse them unless the url field is there (it can be an empty string)

This change makes sure the media viewer opens the sticker using the thumbnail url (if present) and it does not open if not (instead of trying to load an empty url and end with an error message)

Type of change

• Feature
• Bugfix
• Technical
• Other :

Content

Use the thumbnail url if the event's main url is empty

Motivation and context

Adding the real url might confuse other clients.

Screenshots / GIFs

Tests

• Bridge a sticker from whatsapp/telegram/signal with the patch that adds the url field
  OR
  Copy the event and send it to a room manually adding the url field
• See the sticker, click it, it opens

Tested devices

• Physical
• Emulator
• OS version(s):

Checklist

• Changes have been tested on an Android device or Android emulator with API 23
• UI change has been tested on both light and dark themes
• Accessibility has been taken into account. See https://github.com/element-hq/element-x-android/blob/develop/CONTRIBUTING.md#accessibility
• Pull request is based on the develop branch
• Pull request includes a new file under ./changelog.d. See https://github.com/element-hq/element-x-android/blob/develop/CONTRIBUTING.md#changelog
• Pull request includes screenshots or videos if containing UI changes
• Pull request includes a sign off
• You've made a self review of your PR

[surakin]

Did I break the Community PR notice? :P

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (4f6c742) 66.97% compared to head (f0baf9e) 66.97%.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #2157   +/-   ##
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  Coverage    66.97%   66.97%           
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  Files         1375     1375           
  Lines        34221    34219    -2     
  Branches      7545     7544    -1     
========================================
  Hits         22919    22919           
+ Misses        7664     7663    -1     
+ Partials      3638     3637    -1     

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[jmartinesp]

LGTM, thanks for the fix!