Fix a case where joinRoom creates a duplicate Room object

[robintown]

When calling MatrixClient.joinRoom with a room alias, the method would create a new Room object, even if you were already present in that room. This changes its behavior to no-op, as the doc comment promises.

Checklist

• Tests written for new code (and old code if feasible)
• Linter and other CI checks pass

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Here's what your changelog entry will look like:

🐛 Bug Fixes

• Fix a case where joinRoom creates a duplicate Room object (#3747).