ERC: Fix ERC's IBuffer format "crash" on killed server buffer

ERC: Fix ERC's IBuffer format "crash" on killed server buffer

* lisp/erc/erc-ibuffer.el (erc-server-name): Fix a crash when
displaying (or updating) an IBuffer buffer using ERC's first IBuffer
format.  This happens when one ERC buffer has its associated server
buffer killed, e.g., voluntarily or automatically after server
disconnection when `erc-kill-server-buffer-on-quit' is set to t.  The
culprit is the "Server" column, which returns nil in this case.
Display "(closed)" instead (bug#44156).
Copyright-paperwork-exempt: yes