Allow /myroomnick to remove display name

[bkil]

Describe the problem

The /myroomnick command will be ignored if it is not followed by a non-blank display name to set.

Describe the solution you'd like

• I'd like to type /myroomnick (or a dedicated command such as /removemyroomnick) to remove my display name in the current room.
• It may be useful to set a display name to JS null or to the empty string in certain edge cases. I think it would still remain intuitive if typing /myroomnick with a space at the end would set the display name string to the empty string. I wouldn't see the ability to set it to null important for my use case at the moment.

Alternatives considered

• Removing the display name globally for the whole account and then individually restoring the display name in all other rooms individually, spamming the timeline for everyone else.
• /myroomnick "" sets my display name to "", but I did not fuzz it further
• Using another client

Additional context

Useful for debugging Cinny issues for example.