Bug : Crash when typing unknown slash command (e.g., /hello)

This PR fixes a crash that occurs when a user types an unknown slash command (e.g., /hello) and presses Enter.

Previously, entering a slash command that does not exist in the client-side commands list caused the application to crash with a TypeError. This PR ensures that the client handles unknown slash commands gracefully and aligns behavior with standard Rocket.Chat expectations.

Steps to Reproduce

1. Open EmbeddedChat.
2. Type /hello (or any unknown slash command) in the chat input.
3. Press Enter.

Observed Behavior

The application crashes with the following error:

Uncaught (in promise) TypeError: Cannot read properties of undefined (reading 'command')
    at CommandsList.js:33
    at HTMLDocument.handleKeyPress (CommandsList.js:54)

Expected Behavior

• The message /hello should be sent to the server.
• The server should respond with No such command: /hello (standard Rocket.Chat behavior).
• At a minimum, the client should not crash.
• Add a defensive check to ensure selectedItem exists before accessing its properties.

Start in CommandsList.js, especially the keypress handling at the reported lines, and reproduce the issue by entering /hello and pressing Enter. Done means unknown commands no longer cause a TypeError, the message is sent for server handling, and the client remains usable.