SDK does not include m.mentions in events without mentions

[krille-chan]

Checklist

• I could not find a solution in the documentation, the existing issues or discussions.
• I already asked for help in the chat

In which Project did the bug appear?

Other

If you selected "Other" as Project, please enter in which project the bug occurred.

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On which platform did the bug appear?

Android, iOS

SDK Version

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Describe the problem caused by this bug

From krille-chan/fluffychat#2731 :

While m.mentions are included in events that explicitly mention users, the entry is completely omitted in events that do not explicitly mention an entity.
This means events that do not mention anyone will trigger legacy mentions, which is undesired behaviour in today's spec: https://spec.matrix.org/v1.17/client-server-api/#user-and-room-mentions:~:text=it%20is%20recommended%20that%20clients%20still%20include%20a%20m.mentions%20property%20on%20each%20event

Steps To Reproduce

1. Send an event with a mention - it includes m.mentions
2. Send an event with someone's localpart - it does not include m.mentions, but still (unintentionally) mentions them

Screenshots or Logs

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Security related

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