Clear buffer-undo-list after sending input in ERC

Clear buffer-undo-list after sending input in ERC

* lisp/erc/erc.el (erc-insert-line): Assume `erc-insert-marker' points
somewhere.
(erc-send-current-line): Set `buffer-undo-list' to nil because it should
only record editing changes in the prompt area, which has just been
cleared.  ERC did this via `erc-display-prompt' prior to 5.6, but it now
leaves the prompt alone by default.
* test/lisp/erc/erc-tests.el (erc-update-undo-list): New test.
(Bug#74518)

(cherry picked from commit e0d2c6f20f01366c41ba33d663a2b319dd9b7ab1)