events(fix): add mentions in the plain edit event when replacing an event with no initial mentions

[bnjbvr]

When creating a replacement event, there are two places where we set mentions, if provided:

• on the event content itself,
• on the m.new_content field.

Mentions are fully included in the m.new_content variant, but for the event content itself, mentions that were already present were filtered out. Unfortunately, this code was no correct when the original event had no mentions at all: in this case, the mentions present in the event content itself would be empty, because of a lack of a special case in the code handling those mentions.

The new test introduced acts as a regression test; it fails on main at line 869, because mentions are set to None there.

Found while working on matrix-org/matrix-rust-sdk#4464.

[zecakeh]

Thanks! Please remove the (fix) in the commit message.