Fix creatorUserId is required error when opening sticker picker

[justinbot]

Fixes this error causing the sticker picker to show blank on Element Web v1.11.25:

rageshake.ts:73 Error: creatorUserId is required
    at t.assertPresent (utils.js:25:11)
    at new e (Widget.js:28:16)
    at new Ln (StopGapWidget.ts:87:9)
    at vs.hasPermissionToLoad (AppTile.tsx:168:76)
    at vs.getNewState (AppTile.tsx:233:39)
    at new vs (AppTile.tsx:149:27)
    at vo (react-dom.production.min.js:135:172)
    at qa (react-dom.production.min.js:181:146)
    at Ws (react-dom.production.min.js:269:427)
    at Ol (react-dom.production.min.js:250:347)

This started happening following #10121 but it was really due to the WidgetUtils.setUserWidget failing to include a creatorUserId all along.

The fixes here are:

1. Fall back to the widget event sender so that existing user widgets which are missing a creatorUserId will work again. (Since this is a user widget, I don't believe creatorUserId would ever be different from the sender anyway, but the RFC says to include it.)
2. Include the creatorUserId when adding user widgets.

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Here's what your changelog entry will look like:

🐛 Bug Fixes

• Fix creatorUserId is required error when opening sticker picker (#10423).

[andybalaam]

This looks good (thank you!), but one question: why did the existing test pass? Is there a way to add a test that would have failed before this fix?

[justinbot]

The existing test passed because it did specify a creatorUserId in the widget event even though in reality WidgetUtils.setUserWidget wouldn't set one.
i.e. The existing test would have failed if the event data was accurate, or somehow set through that util instead of manually.

[andybalaam]

Covered by Cypress tests, so merging despite coverage check failure.