Fix regression in erc-send-input-line

* lisp/erc/erc.el (erc-send-input-line): Restore remedial single-space
padding to ensure empty messages typed at the prompt without an
explicit /msg aren't rejected by the server.  This behavior is only
noticeable when `erc-send-whitespace-lines' is active.

* test/lisp/erc/erc-tests.el (erc-process-input-line): Add trailing
newline to more correctly simulate how it's actually called by
`erc-send-input'. (Bug#50008)

1 files changed, 2 insertions, 0 deletions

diff --git a/lisp/erc/erc.el b/lisp/erc/erc.el
index 1c221a9cb13..971d3f426fc 100644
--- a/lisp/erc/erc.el
+++ b/lisp/erc/erc.el
@@ -2992,6 +2992,8 @@ for special purposes (see erc-dcc.el).")
 
 (defun erc-send-input-line (target line &optional force)
   "Send LINE to TARGET."
+  (when (string= line "\n")
+    (setq line " \n"))
   (erc-message "PRIVMSG" (concat target " " line) force))
 
 (defun erc-get-arglist (fun)