/[room]nick needs an undo

Ideally, just '/nick' or '/roomnick' without arguments should remove the room nick. The other option - assiging the empty name - is not useful, and the current error message doesn't bring anything either.

Locate the /[room]nick command handling in the Element iOS codebase and first inspect how no-argument commands and room nicknames are currently processed. Done means that /nick or /roomnick without arguments removes the room nick instead of producing the current error; check for existing command tests and update or run them if present.

Written by the indexing model from the issue text.